Abstract: It is possible to build automatic checkoot and fault diagnosis expert system of naval gun by use of the method corresponding with the fauh diagnosis idea of the maintenance specialist, i. e. the layer-modularized fault diagnosis method. The system formation and functional principle, the knowledge representation of checkout and diagnosis rules, the layer-modullarized diagnosis reasoning mechanism, the explanation mechanism and learning function are described. Consequently, the basis for developing a fauh diagnosis system of new naval gun is provided.