Abstract: The fault diagnosis device for a gun servo system was developed, including the choice of test points, hardware design, signal preprocessing and systems designs for the methods and tactics of diagnosis. The paper also presents diagnosis schemes for certain and fuzzy troubles. Distributed structure and expert system have been applied to the diagnosis apparatus. Test results of this device covered 85% diagnosis for servo systems with a trouble shooting success rate reaching 96%, and taking a diagnosis time of less than 5 min.