The constituting of diagnosis expert system and the basic theory of fault tree analysis were introduced at first. A model of combining fault tree with expert system was analyzed. It focused on the hydraulic excavator PC220-7 for Komatsu according to the hydraulic system breakdown of law and order was particularity. A method combined the fault tree analysis with an expert system based rules was adopted. It was expressed systematically expert knowledge by building a fault tree. According to the fault tree, a database of diagnostic rule can be automatically generated. Automatic diagnosis knowledge acquisition has become reality. A hydraulic system fault of excavator was given as an example to design an expert system with the function of complex reasoning and explanation. It provides a reference for hydraulic excavator of the use of management and fault maintenance personnel to solve excavator problems. It shows that this method has a good application in the future.