The invention includes a method, system and device for switching across packet domain based on IP multimedia subsystem, the invention's application make that multi mode terminal do not need to register to IMS network integrallty once more after it switched to the switching target packet domain, and that the across packet domain switching rate for multimedia real-time session be improved effectively. And, in the process of switching, the flows of interaction signals between terminal and network do not need relating to the S-CSCF and SIP AS of IMS core network, which improve the switching efficiency of the multimedia real-time session.