A method for acquiring a key by a multi-system radio access network, comprising: when a user equipment performs initial access, an authentication message parse and judgment module of an initial radio access network in the multi-system radio access network receiving and parsing an authentication message sent by a multi-system core network, and a key judgment module of the initial radio access network judging whether a ciphering key used by the initial radio access network and sent by the multi-system core network subsequently is an initial value of the ciphering key generated by Home Location Register/Authentication Center, HLR/AuC, according to a parsing result; and if judging that the key sent by the multi-system core network is the initial value, when the user equipment performs multi-system handover from the initial radio access network to a target radio access network in the multi-system radio access network, a key conversion module of the initial radio access network performing key conversion on the ciphering key to obtain a target system key, which is transmitted to the target radio access network; wherein, the multi-system radio access network comprises a Radio Network Controller,RNC, and a Base Station Controller,BSC, and both the RNC and BSC comprise the authentication message parse and judgment module, key judgment module and key conversion module, wherein: the step of judging whether the key sent by the multi-system core network subsequently is the initial value according to a parsing result comprises: when the user equipment performs the initial access through the RNC, and the authentication message is a quintuplet authentication message, or, when the user equipment performs the initial access through the BSC, and the authentication message is not the quintuplet authentication message, judging that the key sent by the multi-system core network subsequently is the initial value. when a user equipment performs initial access, an authentication message parse and judgment module of an initial radio access network in the multi-system radio access network receiving and parsing an authentication message sent by a multi-system core network, and a key judgment module of the initial radio access network judging whether a ciphering key used by the initial radio access network and sent by the multi-system core network subsequently is an initial value of the ciphering key generated by Home Location Register/Authentication Center, HLR/AuC, according to a parsing result; and if judging that the key sent by the multi-system core network is the initial value, when the user equipment performs multi-system handover from the initial radio access network to a target radio access network in the multi-system radio access network, a key conversion module of the initial radio access network performing key conversion on the ciphering key to obtain a target system key, which is transmitted to the target radio access network; wherein, the multi-system radio access network comprises a Radio Network Controller,RNC, and a Base Station Controller,BSC, and both the RNC and BSC comprise the authentication message parse and judgment module, key judgment module and key conversion module, wherein: the step of judging whether the key sent by the multi-system core network subsequently is the initial value according to a parsing result comprises: when the user equipment performs the initial access through the RNC, and the authentication message is a quintuplet authentication message, or, when the user equipment performs the initial access through the BSC, and the authentication message is not the quintuplet authentication message, judging that the key sent by the multi-system core network subsequently is the initial value.