The present application relates to mobile communication technical field, and a method for optimizing activation/deactivation Idle State signaling Reduction (ISR) and a network side device are disclosed. The method includes: during a process of updating User Element (UE) area, a first network entity, when receiving a context request message transmitted from a second entity, determines to activate or deactivate ISR, according to a mobile-reachable timer or an implicit detach timer corresponding to said UE, or operation states of the mobile-reachable timer and the implicit detach timer; and the first network entity transmits the determination result to the second network entity, said second network entity determines to instruct the UE to activate or deactivate the ISR according to the received determination result. The application optimizes the existing procedure for the activating/deactivating of the ISR, enables the ISR processing at the network side and the UE side to be consistent in height, and thus avoiding unnecessary signaling expense.