diff --git a/app/(admin)/settings/admin-settings-client.tsx b/app/(admin)/settings/admin-settings-client.tsx
index 4f9a3ac..155c9ec 100644
--- a/app/(admin)/settings/admin-settings-client.tsx
+++ b/app/(admin)/settings/admin-settings-client.tsx
@@ -151,6 +151,7 @@ type SiteSettings = {
   usageTimeStart: string;
   usageTimeEnd: string;
   usageTimeMessage: string;
+  defaultTheme: "light" | "dark" | "system";
 };
 
 type NavLink = {
@@ -340,6 +341,7 @@ export function AdminSettingsClient() {
     usageTimeStart: "08:00",
     usageTimeEnd: "20:00",
     usageTimeMessage: "由壹点累了也是需要休息的",
+    defaultTheme: "system",
   });
 
   useEffect(() => {
@@ -354,6 +356,7 @@ export function AdminSettingsClient() {
         usageTimeStart: siteSettingsData.usageTimeStart || "08:00",
         usageTimeEnd: siteSettingsData.usageTimeEnd || "20:00",
         usageTimeMessage: siteSettingsData.usageTimeMessage || "由壹点累了也是需要休息的",
+        defaultTheme: siteSettingsData.defaultTheme || "system",
       });
     }
   }, [siteSettingsData]);
@@ -2281,6 +2284,25 @@ export function AdminSettingsClient() {
                   placeholder="https://example.com/favicon.ico"
                 />
               </div>
+              <div className="space-y-2">
+                <Label htmlFor="defaultTheme">默认主题模式</Label>
+                <Select
+                  value={siteSettings.defaultTheme}
+                  onValueChange={(value: "light" | "dark" | "system") => setSiteSettings({ ...siteSettings, defaultTheme: value })}
+                >
+                  <SelectTrigger id="defaultTheme">
+                    <SelectValue placeholder="选择默认主题" />
+                  </SelectTrigger>
+                  <SelectContent>
+                    <SelectItem value="system">跟随系统</SelectItem>
+                    <SelectItem value="light">浅色模式</SelectItem>
+                    <SelectItem value="dark">深色模式</SelectItem>
+                  </SelectContent>
+                </Select>
+                <div className="text-xs text-muted-foreground">
+                  设置新用户首次访问时的默认主题模式
+                </div>
+              </div>
               <div className="flex items-center justify-between space-x-2 rounded-lg border p-4">
                 <div className="space-y-0.5">
                   <Label htmlFor="showAutoModel">显示"自动"模型选项</Label>
diff --git a/app/api/site-settings/route.ts b/app/api/site-settings/route.ts
index 5dac60b..646e341 100644
--- a/app/api/site-settings/route.ts
+++ b/app/api/site-settings/route.ts
@@ -26,6 +26,7 @@ export async function GET() {
         siteLogoDark: null,
         favicon: null,
         showAutoModel: true,
+        defaultTheme: "system",
         usageTimeEnabled: false,
         usageTimeStart: "08:00",
         usageTimeEnd: "20:00",
@@ -58,6 +59,7 @@ const updateSchema = z.object({
   siteLogoDark: z.string().nullable().optional(),
   favicon: z.string().nullable().optional(),
   showAutoModel: z.boolean().optional(),
+  defaultTheme: z.enum(["system", "light", "dark"]).optional(),
   usageTimeEnabled: z.boolean().optional(),
   usageTimeStart: z.string().length(5).optional(),
   usageTimeEnd: z.string().length(5).optional(),
@@ -106,6 +108,7 @@ export async function PUT(request: NextRequest) {
           siteLogoDark: data.siteLogoDark,
           favicon: data.favicon,
           showAutoModel: data.showAutoModel ?? true,
+          defaultTheme: data.defaultTheme ?? "system",
           usageTimeEnabled: data.usageTimeEnabled ?? false,
           usageTimeStart: data.usageTimeStart ?? "08:00",
           usageTimeEnd: data.usageTimeEnd ?? "20:00",
diff --git a/app/layout.tsx b/app/layout.tsx
index e6af49c..0d4e294 100644
--- a/app/layout.tsx
+++ b/app/layout.tsx
@@ -125,11 +125,15 @@ const CHUNK_ERROR_RECOVERY_SCRIPT = `\
   });
 })();`;
 
-export default function RootLayout({
+export default async function RootLayout({
   children,
 }: Readonly<{
   children: React.ReactNode;
 }>) {
+  // 获取网站设置中的默认主题
+  const settings = await getSiteSettings();
+  const defaultTheme = settings.defaultTheme || "system";
+  
   return (
     <html
       className={`${geist.variable} ${geistMono.variable}`}
@@ -158,7 +162,7 @@ export default function RootLayout({
         <FaviconUpdater />
         <ThemeProvider
           attribute="class"
-          defaultTheme="dark"
+          defaultTheme={defaultTheme}
           disableTransitionOnChange
           enableSystem
         >
diff --git a/lib/db/queries.ts b/lib/db/queries.ts
index ec646b5..5cc0ba2 100644
--- a/lib/db/queries.ts
+++ b/lib/db/queries.ts
@@ -1510,9 +1510,9 @@ export async function getSubModelsByParentModelId(parentModelId: string) {
 export async function getSiteSettings() {
   try {
     const [settings] = await db.select().from(siteSettings).limit(1);
-    return settings || { siteName: "AI Chatbot", siteLogo: null, siteLogoDark: null, favicon: null };
+    return settings || { siteName: "AI Chatbot", siteLogo: null, siteLogoDark: null, favicon: null, defaultTheme: "system" };
   } catch (_error) {
-    return { siteName: "AI Chatbot", siteLogo: null, siteLogoDark: null, favicon: null };
+    return { siteName: "AI Chatbot", siteLogo: null, siteLogoDark: null, favicon: null, defaultTheme: "system" };
   }
 }
 
diff --git a/lib/db/schema.ts b/lib/db/schema.ts
index 2b06fb7..7dbbb13 100644
--- a/lib/db/schema.ts
+++ b/lib/db/schema.ts
@@ -286,6 +286,7 @@ export const siteSettings = pgTable("SiteSettings", {
   siteLogoDark: text("siteLogoDark"), // 黑夜模式 LOGO
   favicon: text("favicon"),
   showAutoModel: boolean("showAutoModel").notNull().default(true), // 是否显示"自动"模型选项
+  defaultTheme: varchar("defaultTheme", { length: 16 }).notNull().default("system"), // 默认主题：system | light | dark
   // 使用时间限制
   usageTimeEnabled: boolean("usageTimeEnabled").notNull().default(false), // 是否启用时间限制
   usageTimeStart: varchar("usageTimeStart", { length: 5 }).default("08:00"), // 开始时间（HH:mm）