In order to realize the performance testing and fault diagnosis of communication devices in the Command and Control System, a testing system is proposed. In this paper, a novel communication protocol is developed to realize the communication between the host computer and lower computer of the testing system. As to the different functions of the testing system, the format and sense of the communication messages were introduced in detail. 15 kinds of messages were designed to complete the functions of the open and close of communication serial, data detection ports, the receive and transmission of detection data, the control of the I/O and so on. The communication protocol makes it possible to form the test path and achieve the test of different devices.