In the digital telephone switching system, designing an IMS access gateway is to solve the applications of voice access to the CM-IMS network. When IP or the TDM relay failed, the IMS access gateway intelligent routing can reconnect automatically. According to the actual configuration, the routing strategy can meet the reliability of communication and lowest cost. This paper introduces the application and research of IMS access gateway and expounds the situation of Asterisk. It describes the function of IMS access gateway intelligent routing and analyzes the design in detail, including the attributes of the routing and the definition of data structure. Finally, I realize the function of intelligent routing in combination with the Asterisk open source project. Choosing relay and number transform are verified by experiments. And the intelligent routing has reached the requirement of reliability and low cost.