Communication network of large automatic control system is composed by many subnets, and the protocols of the subnets are generally different from each other. So it makes poor expansibility and cost too much in maintenance and modification. This paper presents a modularized software design of communication gateway, which converts each communication protocol to an internal protocol to achieve intercommunication, so that the core module is irrelative from protocol. While new protocol is brought in, it just needs to add the new interface and the protocol transferring function. The application in control system demonstrates this gateway is extendable and flexible.