Abstract: Under theoritical study and analysis signal of gear vibration by various processing methods, some effective criteria are summed up and the expert system is set up. In this system, in order to make uncertain reasoning, fuzzy theory is applied to knowledge presentation;the backward reasoning mechanism controlled by meta-knowledge is adopted; the reasenable explanation according to the reasoning result is attached as well, The other features of this system are:1) not only being useful for automitic diagnosis by itself but used for pricise diagnosis cooperated with other methods, and 2) having consideration of both on-line monitoring and off-line analysis at the same time.