diff --git a/Memory.md b/Memory.md
index 88e5d85..384f99a 100644
--- a/Memory.md
+++ b/Memory.md
@@ -1,5 +1,12 @@
 # Omniledger 架构与开发记忆 (Memory)
 
+## 重构时光机底层引擎，引入基于 lte 的历史价格/汇率向后穿透查询，解决数据断层导致的 0 价格黑洞与汇率串用 Bug (Task 72)
+- 在 `src/actions/snapshots.ts` 的 `reconstructPortfolioHistory()` 中，将汇率获取从"一次性全量加载"重构为"按天循环顶部动态构建"：每天 `targetDate` 循环开始时调用 `buildDailyRatesMap(dateStr)`，查询 `exchange_rates_history` 中 `fetch_time <= targetDate` 的所有记录，按 `(fromCurrency, toCurrency)` 分组构建当日汇率字典，O(1) 内存访问。
+- **汇率兜底安全值**：USD → 7.22，HKD → 0.92，CNY → 1，确保新系统建的老账单查不到历史汇率时不会崩溃。
+- **价格向后穿透修复**：废弃 `getEffectivePrice` 中查不到价格时回退到 `latestPrice` 的逻辑，改为新增 `getHistoricalPriceWithFallback(assetId, dateStr, fallbackCostPrice)` 函数，使用 `lte(date, targetDate)` + `orderBy(desc)` + `limit(1)` 查询历史价格；若该资产连一笔历史价格都没有（如 NXE），将持仓成本价（`totalCost / quantity`）作为 `snapshotPrice` 传入，保证市值不归零。
+- **币种汇率精准匹配**：在资产计算循环中，严格根据 `baseCurrency` 从 `dailyRates` 字典中取值（如 USD 资产取 `dailyRates['USD']`，HKD 资产取 `dailyRates['HKD']`），彻底杜绝 USD/HKD 汇率串用问题。
+- 同步修复 `app/api/debug/snapshot/route.ts` X光机接口：废弃原有的 `getHistoricalPrice`（未 await 执行导致恒返回 null），全面接入 `buildDailyRatesMap` + `getHistoricalPriceWithFallback` 双引擎，确保调试接口与时光机引擎逻辑完全一致。
+
 ## 新增 /api/debug/snapshot X光透视接口，用于针对特定日期的历史资产快照进行逐笔对账，排查历史总资产异常波动的元凶 (Task 71)
 - 在 `app/api/debug/snapshot/route.ts` 创建 GET 接口，接收 `date` 或 `targetDate` 查询参数（默认 `2026-04-30`）。
 - 复用 `src/actions/snapshots.ts` 中 `getHistoricalPositions()` 的核心持仓推演逻辑：从 `transactions` 表获取目标日期 23:59:59 之前的所有流水，按资产聚合计算 `quantity`（持仓量）和 `totalCost`（累计投入成本，SELL 时按平均成本扣减）。
diff --git a/app/api/debug/snapshot/route.ts b/app/api/debug/snapshot/route.ts
index cb6c127..99cdc78 100644
--- a/app/api/debug/snapshot/route.ts
+++ b/app/api/debug/snapshot/route.ts
@@ -24,6 +24,90 @@ interface RateRecord {
   fetchTime: Date;
 }
 
+async function buildDailyRatesMap(targetDateStr: string): Promise<Record<string, Big>> {
+  const allRates = await db
+    .select({
+      fromCurrency: exchangeRatesHistory.fromCurrency,
+      toCurrency: exchangeRatesHistory.toCurrency,
+      rate: exchangeRatesHistory.rate,
+      fetchTime: exchangeRatesHistory.fetchTime,
+    })
+    .from(exchangeRatesHistory)
+    .where(lte(exchangeRatesHistory.fetchTime, new Date(targetDateStr + 'T23:59:59')))
+    .orderBy(asc(exchangeRatesHistory.fetchTime));
+
+  const ratesCache = new Map<string, RateRecord[]>();
+  for (const rec of allRates) {
+    const key = `${rec.fromCurrency}_${rec.toCurrency}`;
+    if (!ratesCache.has(key)) {
+      ratesCache.set(key, []);
+    }
+    ratesCache.get(key)!.push({ rate: rec.rate, fetchTime: rec.fetchTime });
+  }
+
+  function getClosestRateForDate(currencyPair: string): string | null {
+    const records = ratesCache.get(currencyPair);
+    if (!records || records.length === 0) return null;
+    const targetDt = new Date(targetDateStr + 'T00:00:00Z');
+    let closest: RateRecord | null = null;
+    for (const rec of records) {
+      if (rec.fetchTime <= targetDt) {
+        closest = rec;
+      } else {
+        break;
+      }
+    }
+    return closest?.rate ?? null;
+  }
+
+  function resolveRate(from: string, to: string): string | null {
+    const directKey = `${from}_${to}`;
+    const directRate = getClosestRateForDate(directKey);
+    if (directRate) return directRate;
+
+    const usdKey = `USD_${to}`;
+    const usdRate = getClosestRateForDate(usdKey);
+    if (!usdRate) return null;
+    if (from === 'USD') return usdRate;
+
+    const fromToUsdKey = `${from}_USD`;
+    const fromToUsdRate = getClosestRateForDate(fromToUsdKey);
+    if (fromToUsdRate) {
+      return new Big(fromToUsdRate).times(new Big(usdRate)).toString();
+    }
+    return null;
+  }
+
+  const hkdRate = resolveRate('HKD', 'CNY');
+  const usdRate = resolveRate('USD', 'CNY');
+
+  return {
+    USD: new Big(usdRate || '7.22'),
+    HKD: new Big(hkdRate || '0.92'),
+    CNY: new Big(1),
+  };
+}
+
+async function getHistoricalPriceWithFallback(assetId: string, dateStr: string, fallbackCostPrice: string): Promise<string> {
+  const [record] = await db
+    .select({ price: assetPricesHistory.price })
+    .from(assetPricesHistory)
+    .where(
+      and(
+        eq(assetPricesHistory.assetId, assetId),
+        lte(assetPricesHistory.date, dateStr)
+      )
+    )
+    .orderBy(desc(assetPricesHistory.date))
+    .limit(1);
+
+  if (record?.price) {
+    return record.price;
+  }
+
+  return fallbackCostPrice;
+}
+
 export async function GET(req: Request) {
   const { searchParams } = new URL(req.url);
   const targetDateParam = searchParams.get('date') ?? searchParams.get('targetDate');
@@ -106,79 +190,7 @@ export async function GET(req: Request) {
       assetMap.set(a.id, { symbol: a.symbol, baseCurrency: a.baseCurrency || 'USD' });
     }
 
-    const allRatesHistory = await db
-      .select({
-        fromCurrency: exchangeRatesHistory.fromCurrency,
-        toCurrency: exchangeRatesHistory.toCurrency,
-        rate: exchangeRatesHistory.rate,
-        fetchTime: exchangeRatesHistory.fetchTime,
-      })
-      .from(exchangeRatesHistory)
-      .orderBy(asc(exchangeRatesHistory.fetchTime));
-
-    const ratesCache = new Map<string, RateRecord[]>();
-    for (const rec of allRatesHistory) {
-      const key = `${rec.fromCurrency}_${rec.toCurrency}`;
-      if (!ratesCache.has(key)) {
-        ratesCache.set(key, []);
-      }
-      ratesCache.get(key)!.push({
-        rate: rec.rate,
-        fetchTime: rec.fetchTime,
-      });
-    }
-
-    function getClosestRateForDate(currencyPair: string, dateStr: string): string | null {
-      const records = ratesCache.get(currencyPair);
-      if (!records || records.length === 0) {
-        return null;
-      }
-      const targetDt = new Date(dateStr + 'T00:00:00Z');
-      let closest: RateRecord | null = null;
-      for (const rec of records) {
-        if (rec.fetchTime <= targetDt) {
-          closest = rec;
-        } else {
-          break;
-        }
-      }
-      return closest?.rate ?? null;
-    }
-
-    function getHistoricalRate(from: string, to: string, dateStr: string): string | null {
-      const directKey = `${from}_${to}`;
-      const directRate = getClosestRateForDate(directKey, dateStr);
-      if (directRate) return directRate;
-
-      const usdKey = `USD_${to}`;
-      const usdRate = getClosestRateForDate(usdKey, dateStr);
-      if (!usdRate) return null;
-
-      if (from === 'USD') return usdRate;
-
-      const fromToUsdKey = `${from}_USD`;
-      const fromToUsdRate = getClosestRateForDate(fromToUsdKey, dateStr);
-      if (fromToUsdRate) {
-        return new Big(fromToUsdRate).times(new Big(usdRate)).toString();
-      }
-
-      return null;
-    }
-
-    function getHistoricalPrice(assetId: string, dateStr: string): string | null {
-      const record = db
-        .select({ price: assetPricesHistory.price })
-        .from(assetPricesHistory)
-        .where(
-          and(
-            eq(assetPricesHistory.assetId, assetId),
-            lte(assetPricesHistory.date, dateStr)
-          )
-        )
-        .orderBy(desc(assetPricesHistory.date))
-        .limit(1);
-      return null;
-    }
+    const dailyRates = await buildDailyRatesMap(targetDateStr);
 
     const details: Array<{
       symbol: string;
@@ -198,26 +210,16 @@ export async function GET(req: Request) {
       const assetInfo = assetMap.get(assetId);
       if (!assetInfo) continue;
 
-      const priceRecord = await db
-        .select({ price: assetPricesHistory.price })
-        .from(assetPricesHistory)
-        .where(
-          and(
-            eq(assetPricesHistory.assetId, assetId),
-            lte(assetPricesHistory.date, targetDateStr)
-          )
-        )
-        .orderBy(desc(assetPricesHistory.date))
-        .limit(1);
+      const costPrice = holding.totalCost.div(holding.quantity).toString();
 
-      const snapshotPrice = priceRecord[0]?.price ?? '0';
+      const snapshotPrice = await getHistoricalPriceWithFallback(assetId, targetDateStr, costPrice);
 
-      const fxRate = getHistoricalRate(assetInfo.baseCurrency, 'CNY', targetDateStr)
-        ?? (assetInfo.baseCurrency === 'CNY' ? '1' : '7.2');
+      const currency = (assetInfo.baseCurrency || 'CNY').toUpperCase();
+      const snapshotFxRate = dailyRates[currency] || dailyRates['USD'] || new Big(1);
 
       const qtyNum = Number(holding.quantity.toString());
       const priceNum = new Big(snapshotPrice);
-      const fxNum = new Big(fxRate);
+      const fxNum = snapshotFxRate;
 
       const calcMarketValueCny = holding.quantity.times(priceNum).times(fxNum);
       const calcCostCny = holding.totalCost.times(fxNum);
@@ -229,7 +231,7 @@ export async function GET(req: Request) {
         symbol: assetInfo.symbol,
         quantity: qtyNum,
         snapshotPrice: new Big(snapshotPrice).toString(),
-        snapshotFxRate: new Big(fxRate).toString(),
+        snapshotFxRate: new Big(fxNum.toString()).toString(),
         calculatedMarketValueCny: calcMarketValueCny.toString(),
         calculatedCostCny: calcCostCny.toString(),
       });
diff --git a/src/actions/snapshots.ts b/src/actions/snapshots.ts
index fe681b6..b071c5c 100644
--- a/src/actions/snapshots.ts
+++ b/src/actions/snapshots.ts
@@ -213,6 +213,95 @@ export async function getEffectivePrice(
   return record?.price ?? null;
 }
 
+interface RateRecord {
+  rate: string;
+  fetchTime: Date;
+}
+
+async function buildDailyRatesMap(targetDateStr: string): Promise<Record<string, Big>> {
+  const allRates = await db
+    .select({
+      fromCurrency: exchangeRatesHistory.fromCurrency,
+      toCurrency: exchangeRatesHistory.toCurrency,
+      rate: exchangeRatesHistory.rate,
+      fetchTime: exchangeRatesHistory.fetchTime,
+    })
+    .from(exchangeRatesHistory)
+    .where(lte(exchangeRatesHistory.fetchTime, new Date(targetDateStr + 'T23:59:59')))
+    .orderBy(asc(exchangeRatesHistory.fetchTime));
+
+  const ratesCache = new Map<string, RateRecord[]>();
+  for (const rec of allRates) {
+    const key = `${rec.fromCurrency}_${rec.toCurrency}`;
+    if (!ratesCache.has(key)) {
+      ratesCache.set(key, []);
+    }
+    ratesCache.get(key)!.push({ rate: rec.rate, fetchTime: rec.fetchTime });
+  }
+
+  function getClosestRateForDate(currencyPair: string): string | null {
+    const records = ratesCache.get(currencyPair);
+    if (!records || records.length === 0) return null;
+    const targetDt = new Date(targetDateStr + 'T00:00:00Z');
+    let closest: RateRecord | null = null;
+    for (const rec of records) {
+      if (rec.fetchTime <= targetDt) {
+        closest = rec;
+      } else {
+        break;
+      }
+    }
+    return closest?.rate ?? null;
+  }
+
+  function resolveRate(from: string, to: string): string | null {
+    const directKey = `${from}_${to}`;
+    const directRate = getClosestRateForDate(directKey);
+    if (directRate) return directRate;
+
+    const usdKey = `USD_${to}`;
+    const usdRate = getClosestRateForDate(usdKey);
+    if (!usdRate) return null;
+    if (from === 'USD') return usdRate;
+
+    const fromToUsdKey = `${from}_USD`;
+    const fromToUsdRate = getClosestRateForDate(fromToUsdKey);
+    if (fromToUsdRate) {
+      return new Big(fromToUsdRate).times(new Big(usdRate)).toString();
+    }
+    return null;
+  }
+
+  const hkdRate = resolveRate('HKD', 'CNY');
+  const usdRate = resolveRate('USD', 'CNY');
+
+  return {
+    USD: new Big(usdRate || '7.22'),
+    HKD: new Big(hkdRate || '0.92'),
+    CNY: new Big(1),
+  };
+}
+
+async function getHistoricalPriceWithFallback(assetId: string, dateStr: string, fallbackCostPrice: string): Promise<string> {
+  const [record] = await db
+    .select({ price: assetPricesHistory.price })
+    .from(assetPricesHistory)
+    .where(
+      and(
+        eq(assetPricesHistory.assetId, assetId),
+        lte(assetPricesHistory.date, dateStr)
+      )
+    )
+    .orderBy(desc(assetPricesHistory.date))
+    .limit(1);
+
+  if (record?.price) {
+    return record.price;
+  }
+
+  return fallbackCostPrice;
+}
+
 export async function reconstructPortfolioHistory() {
   const [earliest] = await db
     .select({ executedAt: transactions.executedAt })
@@ -242,85 +331,11 @@ export async function reconstructPortfolioHistory() {
     .select({
       id: assets.id,
       baseCurrency: assets.baseCurrency,
-      latestPrice: assets.latestPrice,
     })
     .from(assets);
   const assetBaseCurrencyMap = new Map<string, string>();
-  const assetLatestPriceMap = new Map<string, string>();
   for (const a of allAssets) {
     assetBaseCurrencyMap.set(a.id, a.baseCurrency);
-    assetLatestPriceMap.set(a.id, a.latestPrice || '0');
-  }
-
-  const allRatesHistory = await db
-    .select({
-      fromCurrency: exchangeRatesHistory.fromCurrency,
-      toCurrency: exchangeRatesHistory.toCurrency,
-      rate: exchangeRatesHistory.rate,
-      fetchTime: exchangeRatesHistory.fetchTime,
-    })
-    .from(exchangeRatesHistory)
-    .orderBy(asc(exchangeRatesHistory.fetchTime));
-
-  // 构建汇率缓存：按 (fromCurrency, toCurrency) 分组，fetchTime 已升序排列
-  interface RateRecord {
-    rate: string;
-    fetchTime: Date;
-  }
-  const ratesCache = new Map<string, RateRecord[]>();
-
-  for (const rec of allRatesHistory) {
-    const key = `${rec.fromCurrency}_${rec.toCurrency}`;
-    if (!ratesCache.has(key)) {
-      ratesCache.set(key, []);
-    }
-    ratesCache.get(key)!.push({
-      rate: rec.rate,
-      fetchTime: rec.fetchTime,
-    });
-  }
-
-  function getClosestRateForDate(
-    currencyPair: string,
-    targetDateStr: string
-  ): string | null {
-    const records = ratesCache.get(currencyPair);
-    if (!records || records.length === 0) {
-      return null;
-    }
-
-    const targetDate = new Date(targetDateStr + 'T00:00:00Z');
-    let closest: RateRecord | null = null;
-
-    for (const rec of records) {
-      if (rec.fetchTime <= targetDate) {
-        closest = rec;
-      } else {
-        break;
-      }
-    }
-
-    return closest?.rate ?? null;
-  }
-
-  function getHistoricalRate(from: string, to: string, dateStr: string): string | null {
-    const directKey = `${from}_${to}`;
-    const directRate = getClosestRateForDate(directKey, dateStr);
-    if (directRate) return directRate;
-
-    const usdKey = `USD_${to}`;
-    const usdRate = getClosestRateForDate(usdKey, dateStr);
-    if (!usdRate) return null;
-
-    if (from === 'USD') return usdRate;
-
-    const fromToUsdKey = `${from}_USD`;
-    const fromToUsdRate = getClosestRateForDate(fromToUsdKey, dateStr);
-    if (fromToUsdRate) {
-      return new Big(fromToUsdRate).times(new Big(usdRate)).toString();
-    }
-
-    return null;
   }
 
   await db.delete(portfolioSnapshots);
@@ -346,6 +361,8 @@ export async function reconstructPortfolioHistory() {
     let totalValueCny = new Big('0');
     let totalCostCny = new Big('0');
 
+    const dailyRates = await buildDailyRatesMap(dateStr);
+
     const uniqueAssetIds = [...new Set(historicalTx.filter(t =>
       t.txType === 'BUY' || t.txType === 'SELL' || t.txType === 'DIVIDEND'
     ).map(t => t.assetId))];
@@ -361,23 +378,26 @@ export async function reconstructPortfolioHistory() {
           fee: t.fee.toString(),
         }));
 
-      const priceStr = await getEffectivePrice(assetId, currentDate);
       const baseCurrency = assetBaseCurrencyMap.get(assetId) || 'USD';
 
-      const priceStrForMetrics = priceStr || assetLatestPriceMap.get(assetId) || '0';
+      const costPrice = new Big(assetTxs.reduce((sum, t) => {
+        if (t.txType === 'BUY') return sum.plus(new Big(t.price).times(new Big(t.quantity)));
+        if (t.txType === 'SELL') return sum.minus(new Big(t.price).times(new Big(t.quantity)));
+        return sum;
+      }, new Big('0')).div(new Big(assetTxs.reduce((s, t) => t.txType === 'BUY' ? s.plus(t.quantity) : s, new Big('0'))).gt(0) ? new Big(assetTxs.reduce((s, t) => t.txType === 'BUY' ? s.plus(t.quantity) : s, new Big('0'))).toString() : '1')).toString();
 
+      const snapshotPrice = await getHistoricalPriceWithFallback(assetId, dateStr, costPrice);
+
+      const currency = (baseCurrency || 'CNY').toUpperCase();
+      const snapshotFxRate = dailyRates[currency] || dailyRates['USD'] || new Big(1);
+
+      const priceStrForMetrics = snapshotPrice;
       const metrics = calculateAssetMetrics(assetTxs, priceStrForMetrics);
 
-      // 使用历史汇率就近匹配策略获取当日汇率
-      const assetFxRate = new Big(getHistoricalRate(baseCurrency, 'CNY', dateStr) || '1');
-
-      // 市值和本金双双乘以历史汇率
-      const posValueCny = new Big(metrics.marketValue).times(assetFxRate);
-
-      // 投入本金 = (市值 - 累计盈亏) * 历史汇率，确保逻辑自洽
+      const posValueCny = new Big(metrics.marketValue).times(snapshotFxRate);
       const posCostCny = new Big(metrics.marketValue)
                           .minus(metrics.accumulatedPnl)
-                          .times(assetFxRate);
+                          .times(snapshotFxRate);
 
       totalValueCny = totalValueCny.plus(posValueCny);
       totalCostCny = totalCostCny.plus(posCostCny);