A method and device for a mobile terminal detaching are provided. The method comprises: upon deactivation, a mobile terminal starts a first timer, sends to a network apparatus deactivation indication information used to indicate that the mobile terminal is deactivated and keeps a registration state which is the same as the state that the mobile terminal was in before powering-off without performing a detach operation, such that the network apparatus starts a second timer to time synchronously with the first timer; and upon expiration of the first timer, the mobile terminal performs detachment operation if the mobile terminal is not reactivated.