The present invention relates to DRX technology and disclosed are a method and device for computing an activation time, which are used for solving the problem that a UE cannot enter an active state at an accurate time when using an extended DRX cycle. The method is: a network side notifying a UE of a serial number of a current SFN cycle in an extended DRX cycle, and the UE computing an activation time of the UE by combining the length of a preset extended DRX cycle in accordance with the serial number. In this way, when the extended DRX cycle is greater than the SFN cycle, the UE can still compute a correct paging time and/or a time of receiving service data, thereby entering the active state at an accurate time, effectively avoiding the situation where paging messages or service data are lost due to computation errors of the UE, then guaranteeing the service QoS of the UE and improving the service performance of the system.