diff --git a/backend/shared/models/account.py b/backend/shared/models/account.py
index 35a62cc..1688fa7 100644
--- a/backend/shared/models/account.py
+++ b/backend/shared/models/account.py
@@ -24,7 +24,7 @@ class Account(Base):
 
     user = relationship("User", back_populates="accounts")
     tasks = relationship("Task", back_populates="account", cascade="all, delete-orphan")
-    signin_logs = relationship("SigninLog", back_populates="account")
+    signin_logs = relationship("SigninLog", back_populates="account", cascade="all, delete-orphan", passive_deletes=True)
 
     def __repr__(self):
         return f"<Account(id={self.id}, weibo_user_id='{self.weibo_user_id}')>"
diff --git a/backend/shared/models/signin_log.py b/backend/shared/models/signin_log.py
index 1100fd0..7ad4cbc 100644
--- a/backend/shared/models/signin_log.py
+++ b/backend/shared/models/signin_log.py
@@ -10,7 +10,7 @@ from .base import Base
 class SigninLog(Base):
     __tablename__ = "signin_logs"
     id = Column(Integer, primary_key=True, autoincrement=True)
-    account_id = Column(String(36), ForeignKey("accounts.id"), nullable=False)
+    account_id = Column(String(36), ForeignKey("accounts.id", ondelete="CASCADE"), nullable=False)
     topic_title = Column(String(100))
     status = Column(String(50), nullable=False)
     reward_info = Column(JSON, nullable=True)
diff --git a/init-db.sql b/init-db.sql
index 5f7d5bc..f1771f4 100644
--- a/init-db.sql
+++ b/init-db.sql
@@ -59,7 +59,7 @@ CREATE TABLE IF NOT EXISTS signin_logs (
     INDEX idx_signin_logs_account_id (account_id),
     INDEX idx_signin_logs_signed_at (signed_at),
     INDEX idx_signin_logs_status (status),
-    FOREIGN KEY (account_id) REFERENCES accounts(id)
+    FOREIGN KEY (account_id) REFERENCES accounts(id) ON DELETE CASCADE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 
 -- Invite codes table
diff --git a/migrate_fix_signin_logs_fk.sql b/migrate_fix_signin_logs_fk.sql
new file mode 100644
index 0000000..7df3841
--- /dev/null
+++ b/migrate_fix_signin_logs_fk.sql
@@ -0,0 +1,21 @@
+-- 修复 signin_logs 外键：删除账号时级联删除签到记录
+-- 用法: mysql -u weibo -p123456 weibo_hotsign < migrate_fix_signin_logs_fk.sql
+
+-- 先查出旧外键名并删除，再重建
+SET @fk_name = (
+    SELECT CONSTRAINT_NAME FROM information_schema.KEY_COLUMN_USAGE
+    WHERE TABLE_SCHEMA = 'weibo_hotsign'
+      AND TABLE_NAME = 'signin_logs'
+      AND COLUMN_NAME = 'account_id'
+      AND REFERENCED_TABLE_NAME = 'accounts'
+    LIMIT 1
+);
+
+SET @sql = CONCAT('ALTER TABLE signin_logs DROP FOREIGN KEY ', @fk_name);
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+ALTER TABLE signin_logs
+    ADD CONSTRAINT fk_signin_logs_account
+    FOREIGN KEY (account_id) REFERENCES accounts(id) ON DELETE CASCADE;