A method for generating a key identity identifier when a user equipment (UE) transfers is disclosed. The method includes the following steps: a mobility management entity (MME) of an evolved UMTS terrestrial radio access network (EUTRAN) sends an identity identifier of an access security management entity key (KSIASME) to a serving general packet radio service support node (SGSN) of a target system when the UE transfers from the EUTRAN to the target system, and both the SGSN and the UE map the KSIASME into a key identity identifier of the target system.