An expert system of fault diagnosis based on fault tree and case is introduced. The system uses CLIPS to achieve the knowledge base and inference engine，and applies C# to realize the manmachine interface and management modules. The structure information of fault tree, the logical relationships among the nodes case knowledge are preserved by fact grammatical structure of CLIPS. The system uses CLIPS as the inference engine to actualize the forward and backward reasoning based on fault tree and the reasoning based on case comprising stringtype attributes and enumtype attributes, in order to improve the accuracy and reliability.