Abstract: A safety, insurance and alarm mechanical lock contains an external lock portion, a multicylinder type lock cylinder portion, and a lock body portion engaging with the lock cylinder portion. The lock cylinder portion comprises a front part including an external cylinder, a middle cylinder and an internal cylinder and a back part including a back external cylinder and a back internal cylinder. A self-locking control mechanism is provided between the front and back lock cylinders which can automatically perform self-locking and alarming when destroying act applied to the lock cylinder from outside takes place. The lock body portion is provided with an internal lock mechanism. Furthermore, in accordance with the various requirement, it can also be provided with an anti-invading mechanism, an alarm mechanism sensing the deformation of the door. The lock of the present invention has overcome many disadvantages of the prior lock and give a great deal of safety to users.