Instant messaging transfer method of IP multimedia subsystem and instant messaging application server

The invention discloses an instant messaging (IM) transfer method of an IP multimedia subsystem (IMS) and an instant messaging application server (AS). The method comprises the following steps that: an IM AS receives a session initiation protocol (SIP) IM sent by calling IMS user equipment (UE); if called IMS UE is found to be unregistered or be not on-line and the called IMS UE signs an IM transfer service, the SIP IM is converted into a short message and a destination number of the short message is a mobile station international integrated service digital network (MSISDN) number of a mobile terminal bound to the called IMS UE; and the short message is forwarded to the mobile terminal by a short message gateway and a short message platform. According to the invention, on the condition that an IMS user is not registered or is not online, an IM can still be received by a bound mobile terminal.