diff --git a/docs/checks/task-11.md b/docs/checks/task-11.md
index 84bac5c..d3775fc 100644
--- a/docs/checks/task-11.md
+++ b/docs/checks/task-11.md
@@ -14,6 +14,7 @@
 - `server/src/handlers/ai.ts`（重写）：配置有效 → 调用 AI 并返回 `source:'ai'`；未配置 → `source:'fallback'`（「AI 尚未配置」）；超时 → `source:'fallback'`（「AI 请求超时」）；其余失败 → `source:'fallback'`（「AI 讲解暂时不可用」）。回退内容为题库解析 + 标签 + 答案，前端可识别。
 - `server/src/schemas/api.ts`：`AiExplainBodySchema.userAnswer` 改为可选（从错题详情进入且无作答历史时省略）。
 - `server/.env.example`（新增）：记录 PORT / DATA_DIR / AI 配置项说明。
+- `server/src/env.ts` + `server.ts`（补充）：引入 dotenv，启动时最先加载 `server/.env`（进程内环境变量优先），确保 `PORT` / `DATA_DIR` / `AI_*` 在模块读取前就位；实测 `server/.env` 中配置 AI 变量后 `aiConfigured=true` 且讲解走 AI，删除 `.env` 后自动回退。
 
 ### 前端
 
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index f654bee..cf9e453 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -50,6 +50,9 @@ importers:
       '@fastify/swagger-ui':
         specifier: ^6.1.1
         version: 6.1.1
+      dotenv:
+        specifier: ^17.4.2
+        version: 17.4.2
       fastify:
         specifier: latest
         version: 5.12.1
@@ -717,6 +720,10 @@ packages:
     resolution: {integrity: sha512-Btj2BOOO83o3WyH59e8MgXsxEQVcarkUOpEYrubB0urwnN10yQ364rsiByU11nZlqWYZm05i/of7io4mzihBtQ==}
     engines: {node: '>=8'}
 
+  dotenv@17.4.2:
+    resolution: {integrity: sha512-nI4U3TottKAcAD9LLud4Cb7b2QztQMUEfHbvhTH09bqXTxnSie8WnjPALV/WMCrJZ6UV/qHJ6L03OqO3LcdYZw==}
+    engines: {node: '>=12'}
+
   entities@7.0.1:
     resolution: {integrity: sha512-TWrgLOFUQTH994YUyl1yT4uyavY5nNB5muff+RtWaqNVCAK408b5ZnnbNAUEWLTCpum9w6arT70i1XdQ4UeOPA==}
     engines: {node: '>=0.12'}
@@ -1808,6 +1815,8 @@ snapshots:
 
   detect-libc@2.1.2: {}
 
+  dotenv@17.4.2: {}
+
   entities@7.0.1: {}
 
   esbuild@0.28.2:
diff --git a/server/.env.example b/server/.env.example
index a4f1520..8a38f9f 100644
--- a/server/.env.example
+++ b/server/.env.example
@@ -3,7 +3,7 @@ PORT=3000
 # DATA_DIR=./data
 
 # AI 讲解（OpenAI 兼容 Chat Completions；不配置时讲解自动回退题库解析）
-# AI_BASE_URL=https://api.openai.com/v1
-# AI_API_KEY=sk-xxxxxxxx
-# AI_MODEL=gpt-4o-mini
+AI_BASE_URL=https://api.deepseek.com
+AI_API_KEY=sk-xxxxxxxx
+AI_MODEL=deepseek-v4-flash
 # AI_TIMEOUT_MS=20000
diff --git a/server/package.json b/server/package.json
index 74a3c5a..32af2f5 100644
--- a/server/package.json
+++ b/server/package.json
@@ -11,6 +11,7 @@
     "@fastify/cors": "latest",
     "@fastify/swagger": "^9.8.1",
     "@fastify/swagger-ui": "^6.1.1",
+    "dotenv": "^17.4.2",
     "fastify": "latest",
     "zod": "latest"
   },
diff --git a/server/src/env.ts b/server/src/env.ts
new file mode 100644
index 0000000..eb7e3e0
--- /dev/null
+++ b/server/src/env.ts
@@ -0,0 +1,11 @@
+import { config } from 'dotenv'
+import { dirname, join } from 'node:path'
+import { fileURLToPath } from 'node:url'
+
+/**
+ * 启动时最先加载 server/.env（进程内已有环境变量优先，dotenv 默认不覆盖）。
+ * 必须在任何读取 process.env 的模块（如 data/files.ts）之前导入，
+ * 否则 DATA_DIR / AI_* 等配置会读到空值。
+ */
+const serverRoot = join(dirname(fileURLToPath(import.meta.url)), '..')
+config({ path: join(serverRoot, '.env') })
diff --git a/server/src/server.ts b/server/src/server.ts
index cfa85e3..8080dd9 100644
--- a/server/src/server.ts
+++ b/server/src/server.ts
@@ -1,3 +1,6 @@
+// 最先加载 server/.env（dotenv），确保 DATA_DIR / PORT / AI_* 在模块读取前就位
+import './env.js'
+
 import { pathToFileURL } from 'node:url'
 import Fastify from 'fastify'
 import cors from '@fastify/cors'