Abstract: This paper introduces the fault diagnostic expert system that includes system structure,function and self-study capacity for gas generator. The built of knowledge base and fulfillment of inference engine are mainly described. The structure of knowledge base is divided into three parts:knowledge in a large range, knowledge in a small range, and the reference-resolving program. The system uses twin inference engine mechanism. Converse precise inference is used in education and demonstraion,which can examine and explain inference process. Converse fuzzy-inference is applied to indusny production, which provides believable inference result.