Provided are an implementing method and a communication system for a traditional terminal user accessing an IMS domain. The method allocates an IMS domain user identification including a public identification and a private identification to a traditional terminal user on an AGF entity controlled by an AGCF entity, and establishes a mapping relation between the IMS domain user identification and the line identification of the user. When receiving a command or message for a certain user terminal sent from the AGF entity, the AGCF entity determines the IMS domain user identification of the user according to the mapping relation and sends a corresponding SIP message to the IMS domain.