diff --git a/src/auth/auth.service.ts b/src/auth/auth.service.ts
index d24a04b..566e0ac 100644
--- a/src/auth/auth.service.ts
+++ b/src/auth/auth.service.ts
@@ -80,33 +80,70 @@ export class AuthService {
     dto: LoginDto,
     ip?: string,
     userAgent?: string,
+  ): Promise<
+    | { accessToken: string; refreshToken: string; expiresIn: number }
+    | { needSelectTenant: true; tenants: { tenantCode: string; tenantName: string }[] }
+  > {
+    if (dto.tenantCode) {
+      return this.loginTenant(dto.tenantCode, dto.username, dto.password, ip, userAgent);
+    }
+    // 未指定租户：按用户名探测（一个用户名可存在于多个租户）
+    const users = await this.prisma.client.user.findMany({
+      where: { username: dto.username, deletedAt: null },
+      select: { tenantCode: true, passwordHash: true, status: true },
+    });
+    const matched: { tenantCode: string; tenantName: string }[] = [];
+    for (const u of users) {
+      if (u.status !== 'enabled') continue;
+      const tenant = await this.prisma.client.tenant.findUnique({ where: { code: u.tenantCode } });
+      if (!tenant || tenant.status !== 'enabled') continue;
+      if (await bcrypt.compare(dto.password, u.passwordHash)) {
+        matched.push({ tenantCode: u.tenantCode, tenantName: tenant.name });
+      }
+    }
+    if (matched.length === 0) {
+      await this.recordLoginLog('', dto.username, 'fail', '账号或密码错误', ip, userAgent);
+      throw new UnauthorizedException('账号或密码错误');
+    }
+    if (matched.length === 1) {
+      return this.loginTenant(matched[0].tenantCode, dto.username, dto.password, ip, userAgent);
+    }
+    return { needSelectTenant: true, tenants: matched };
+  }
+
+  /** 指定租户登录（单租户逻辑）。 */
+  private async loginTenant(
+    tenantCode: string,
+    username: string,
+    password: string,
+    ip?: string,
+    userAgent?: string,
   ): Promise<{ accessToken: string; refreshToken: string; expiresIn: number }> {
-    const tenant = await this.prisma.client.tenant.findUnique({ where: { code: dto.tenantCode } });
+    const tenant = await this.prisma.client.tenant.findUnique({ where: { code: tenantCode } });
     if (!tenant || tenant.status !== 'enabled') {
-      await this.recordLoginLog(dto.tenantCode, dto.username, 'fail', '租户不存在或已停用', ip, userAgent);
+      await this.recordLoginLog(tenantCode, username, 'fail', '租户不存在或已停用', ip, userAgent);
       throw new UnauthorizedException('租户不存在或已停用');
     }
     const user = await this.prisma.client.user.findFirst({
-      where: { tenantCode: dto.tenantCode, username: dto.username, deletedAt: null },
+      where: { tenantCode, username, deletedAt: null },
     });
     if (!user) {
-      await this.recordLoginLog(dto.tenantCode, dto.username, 'fail', '账号或密码错误', ip, userAgent);
+      await this.recordLoginLog(tenantCode, username, 'fail', '账号或密码错误', ip, userAgent);
       throw new UnauthorizedException('账号或密码错误');
     }
     if (user.status !== 'enabled') {
-      await this.recordLoginLog(dto.tenantCode, dto.username, 'fail', '账号已停用或锁定', ip, userAgent);
+      await this.recordLoginLog(tenantCode, username, 'fail', '账号已停用或锁定', ip, userAgent);
       throw new UnauthorizedException('账号已停用或锁定');
     }
-    const ok = await bcrypt.compare(dto.password, user.passwordHash);
+    const ok = await bcrypt.compare(password, user.passwordHash);
     if (!ok) {
-      await this.recordLoginLog(dto.tenantCode, dto.username, 'fail', '账号或密码错误', ip, userAgent);
+      await this.recordLoginLog(tenantCode, username, 'fail', '账号或密码错误', ip, userAgent);
       throw new UnauthorizedException('账号或密码错误');
     }
-
-    await this.writePermissionCache(dto.tenantCode, user.id);
-    await this.recordLoginLog(dto.tenantCode, dto.username, 'success', null, ip, userAgent);
+    await this.writePermissionCache(tenantCode, user.id);
+    await this.recordLoginLog(tenantCode, username, 'success', null, ip, userAgent);
     await this.recordAudit({
-      tenantCode: dto.tenantCode,
+      tenantCode,
       userId: user.id,
       username: user.username,
       module: 'auth',
@@ -115,7 +152,7 @@ export class AuthService {
       ip: ip ?? null,
       userAgent: userAgent ?? null,
     });
-    return this.issue(dto.tenantCode, user.id, user.username, user.tokenVersion);
+    return this.issue(tenantCode, user.id, user.username, user.tokenVersion);
   }
 
   /** 注册：在指定租户下创建普通用户（无角色，状态启用）。 */
diff --git a/src/auth/dto.ts b/src/auth/dto.ts
index 97dd328..a63739b 100644
--- a/src/auth/dto.ts
+++ b/src/auth/dto.ts
@@ -2,10 +2,10 @@ import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';
 import { IsNotEmpty, IsOptional, IsString } from 'class-validator';
 
 export class LoginDto {
-  @ApiProperty({ description: '租户编码' })
+  @ApiPropertyOptional({ description: '租户编码（多租户二选一时传入；缺省按用户名自动探测）' })
+  @IsOptional()
   @IsString()
-  @IsNotEmpty()
-  tenantCode!: string;
+  tenantCode?: string;
 
   @ApiProperty({ description: '用户名' })
   @IsString()