The fault diagnosis technology of hydropower units is the basis of condition based maintenance of hydroelectric power plant. As a kind of large rotating machinery, the vibration of Hydropower units can reflect most of the units fault. Eexpert system is a relatively mature technology in the field of artificial intelligence, and the fault diagnosis expert system is gradually applied to hydroelectric power plant in domestic. Basing on the condition monitoring of hydropower units, a fault diagnosis expert system baced on production rules for the vibration fault of hydroelectric unit is put forward. The system mainly consists of knowledge base, inference engine, database, human-computer interface and other components. Finally, get the diagnosis results with actual data, and then validate the diagnosis results based on the maintenance report of the hydropower units. The conclusion proved that the diagnostic results of expert system can provide may causes and locations of faults to the staff, and can provide some guidence for the maintenance of Hydropower units.