Embodiments of the present invention provide a handover method, base station, user equipment, and mobility management entity. The handover method includes: obtaining an algorithm used by a user equipment in a universal mobile telecommunication system and four least significant bits of a downlink non-access stratum count; and sending a handover command to the user equipment, where the handover command carries the algorithm used by the user equipment in the universal mobile telecommunication system and the four least significant bits of the downlink non-access stratum count, so that the user equipment calculates a cipher key and an integrity key according to the four least significant bits of the downlink non-access stratum count.