According to the rules on knowledge base, testing personnel can use a variety of diagnostic information when one failure occurs in radar, in order to determine the failure mode of the system by reasoning and implement the fault location and troubleshooting quickly and accurately. Firstly, the framework of intelligent diagnostics base on knowledge reasoning is built, then creation, structure and updating of knowledge base are analyzed, and then the diagnostics model and reasoning mechanisms are analyzed and the diagnostics reasoning process is given. Finally, the demonstration of intelligent diagnostics is carried out by one failure case in naval gun radar.