The invention discloses a method of realizing media switching in a replacement function, which is applied in the communication technical field. The method includes the following steps: a third-party user of an IP multimedia subsystem domain sends a call request for a new session two to a media gateway control network element for replacing the session one between a first user of a circuit-switcheddomain and a second user of the IP multimedia subsystem domain; according to the call request of the session two, the IP multimedia gateway control network element makes a bearing switch of the session two replacing the session one under the control of the media gateway control network element; the first user and the third user enter a steady session state of session two. By the method, the bearing switching of media channels among the first, the second and the third users by the interworking of the IP multimedia subsystem domain and the circuit-switched domain through the media gateway control network elements and the IP multimedia gateway control network elements.