An example of the rule-based expert system applied to the fan fault diagnosis is presented. The architecture and function of the fault diagnosis system are introduced. The expression of the fault diagnosis knowledge and the attribute of knowledge base based on the relational database have been studied. The hybrid reasoning technology was applied to the implementation of the diagnosis inference engine in the expert system. The presented fault diagnosis system is easy to modify the knowledge base with the experience accumulated in practice, and it has the advantages of expansibility, portability, concision, and high efficiency.