你需要，

• 删除现有的外键约束，
• 在启用 ON DELETE CASCADE 设置的情况下添加一个新的.

• Drop the existing foreign key constraint,
• Add a new one with the ON DELETE CASCADE setting enabled.

类似于:

ALTER TABLE dbo.T2
   DROP CONSTRAINT FK_T1_T2   -- or whatever it's called

ALTER TABLE dbo.T2
   ADD CONSTRAINT FK_T1_T2_Cascade
   FOREIGN KEY (EmployeeID) REFERENCES dbo.T1(EmployeeID) ON DELETE CASCADE