A general framework of hydraulic fault diagnosis system was studied. It consisted of equipment knowledge bases, databases, fusion reasoning, knowledge acquisition and so on. The tree-structure model of the fault knowledge was established based on fault hierarchy and logicality. Fault nodes knowledge was encapsulated by object-oriented technique. Complete knowledge bases were made including fault bases and diagnosis bases. It could describe the fault positions, system structure, cause-symptom relationships, diagnosis principles and other knowledge. The results show that the methods are effective.