A charging system switching method, comprising: determining, by an online charging system, OCS, a service for which a charging system needs to be switched, and a target charging system to which the service needs to be switched; characterized in that the method further comprises: constructing, by the OCS, a re-authentication request external session message, wherein the re-authentication request external session message comprises a first identifier and a second identifier, the first identifier is used to indicate the service for which a charging system needs to be switched, and the second identifier is used to indicate the target charging system to which the service needs to be switched; and sending, by the OCS, the re-authentication request external session message to an access device, so as to instruct the access device to switch the service from a current charging system to the target charging system according to the re-authentication request external session message; wherein the determining, by an OCS, a service for which a charging system needs to be switched comprises: determining, by the OCS, a user for which a charging system needs to be switched, and determining a service that is of the user and for which a charging system needs to be switched; wherein if the OCS determines that a charging system needs to be switched for all services of the user, the first identifier is an identifier of the user. determining, by an online charging system, OCS, a service for which a charging system needs to be switched, and a target charging system to which the service needs to be switched; characterized in that the method further comprises: constructing, by the OCS, a re-authentication request external session message, wherein the re-authentication request external session message comprises a first identifier and a second identifier, the first identifier is used to indicate the service for which a charging system needs to be switched, and the second identifier is used to indicate the target charging system to which the service needs to be switched; and sending, by the OCS, the re-authentication request external session message to an access device, so as to instruct the access device to switch the service from a current charging system to the target charging system according to the re-authentication request external session message; constructing, by the OCS, a re-authentication request external session message, wherein the re-authentication request external session message comprises a first identifier and a second identifier, the first identifier is used to indicate the service for which a charging system needs to be switched, and the second identifier is used to indicate the target charging system to which the service needs to be switched; and sending, by the OCS, the re-authentication request external session message to an access device, so as to instruct the access device to switch the service from a current charging system to the target charging system according to the re-authentication request external session message; wherein the determining, by an OCS, a service for which a charging system needs to be switched comprises: determining, by the OCS, a user for which a charging system needs to be switched, and determining a service that is of the user and for which a charging system needs to be switched; wherein if the OCS determines that a charging system needs to be switched for all services of the user, the first identifier is an identifier of the user. determining, by the OCS, a user for which a charging system needs to be switched, and determining a service that is of the user and for which a charging system needs to be switched; wherein if the OCS determines that a charging system needs to be switched for all services of the user, the first identifier is an identifier of the user.