Embodiments of the present invention disclose a system and a method for processing a packet domain service signal, which enable a terminal that does not support an access control protocol of an Internet Protocol Multimedia Subsystem (IMS) to access the IMS and acquire the services in the IMS. An AGCF is added for shielding the differences of the users on the basis of the IMS defined in the 3GPP R5 / R6 standard, specifically, a PSTN / ISDN user accesses an AGF first, accesses the IMS through an agent function of the AGCF, and the AGCF also manages and controls the AGF. Control of the PSTN / ISDN services may be implemented in the AGCF, or in an AS added to the IMS, or the control of basic services and supplementary services may be implemented in the AGCF while the control of value-added services may be implemented in the AS.