Disclosed is a method for notifying the state of a user to an Application Server (AS), including: receiving, by a Serving-Call Session Control Function (S-CSCF), new service profile information of the user sent from a Home Subscriber Server (HSS) when the user's service profile changes; and sending, by the S-CSCF, registration information of the user to the AS. The disclosure also relates to a system for notifying the state of a user to an AS, the system including an HSS, an S-CSCF and the AS. The disclosure also provides a corresponding S-CSCF, a system for notifying the state of a user to an AS, and a corresponding AS. According to the invention, an AS may obtain the registration state of a user timely.