The invention provides a safety device for a brake disk lock, which consists of a brake disk body, a driving ring and a lock body. The lock body is internally provided with a locking cam, a lock core and a spring, wherein, the a lock beam is arranged at the locking cam; the front end of the lock beam is provided with a lock tongue; the brake disk body is provided with a hole; the lock beam or thelock tongue is provided with a safety opening; a safety block and the spring are arranged between the brake disk body and the lock beam or the lock tongue; a button is fixedly connected with the safety block by the hole of the brake disk body; the safety block matches the safety opening. When the safety device is unlocked, the lock tongue can not enter a locking hole in any condition, thus being extremely safe. The safety device has the advantages of simple structure and convenient use and is suitable for being installed in various vehicles using the brake disk.