A method for generating a key identity identifier when a UE (user equipment) transfers, including the following steps: when a UE transfers from an EUTRAN (evolved UMTS terrestrial radio access network) to a target system, an MME (mobility management entity) of the EUTRAN sending a KSIASME (an identity identifier of an access security management entity key (KASME)) to an SGSN (serving GPRS support node) of the target system, and both the SGSN and the UE mapping the KSIASME into a key identity identifier of the target system.
