The invention discloses a registration method and device for an IMS system. The device comprises user equipment UE, a sending unit, a receiving unit, an authentication unit and a registration unit. The method is different from an original IMS registration method. According to the method, when a registration event occurs, a network device needs an access certificate for each user, so the pressure of a server can be mitigated. The certificates are produced by one interface which is not displayed in the network device or an image. When a self-defined condition is satisfied, the network device generates a notice to an enabler, and the enabler sends the notice to the users. Due to connection with P-CSCF, S-CSCF and HSS, the invented network device looks more like a user. The network device is connected with the P-CSCF through an interface Gm, and through utilization of an SIP protocol in the interface, information can be transmitted between the user equipment and a session control function server.