The present invention provides a method, a device, and a system for realizing a registration mechanism of an IP multimedia subsystem. In the above method, a S-CSCF obtains a private user identity and a public user identity of CS UE and characteristic information of a MSC Server from a received request message, wherein the request message includes a registration request or a de-registration request; and then matches the obtained private user identity, public user identity, and characteristic information with existing registration binding relationships. If the match is successful, an operation indicated by said request message is performed on the successfully matched registration binding relationship. Based on the solution proposed in the present invention, the logic of the registration service can be ensured to be normal.