Embodiments of the present invention relate to the field of communications and provide a session transfer method, an application server, and a communications system, which implement a cross-network session transfer between a CS domain and an IMS domain. The method includes: after receiving a session transfer request sent by user B or user C in the IMS domain, sending a release message to the user B currently in a call to terminate a session between user A in the CS domain and the user B, and negotiating media information of the user A and media information of the user C to establish a session between the user A and the user C. The embodiments of the present invention apply to a cross-network session between the CS domain and the IMS domain.