Embodiments of the present invention disclose a session processing method, device, and communication system. The session processing method includes: when a first User Equipment (UE) is to set up an IP Multimedia Subsystem (IMS) session with a second UE, triggering setup of an unstructured supplementary service data (USSD) transaction between a Circuit Switched (CS) network and the first UE; and sending, by using the USSD transaction, an I1 protocol message used to bear IMS session control signaling. With the technical solutions of the present invention, when a UE is to set up an IMS session, a USSD transaction is set up between the UE and the CS network; I1 protocol messages exchanged between the UE and the Service Centralization and Continuity (SCC) Application Server (AS) (SCC-AS) are sent reliably by using the USSD transaction, which facilitates reliable control over IMS services.