The hydraulic control principle and movement process of the injection molding machine were introduced. The Fault Tree Analysis (FTA) method was used to diagnose system failure of the injection molding machine,through analyzing layer by layer and logical relationship between the faults. Practice has proved that FTA method can efficiently and rapidly diagnose faults of the injection molding machine and other complex equipment. Moreover FTA can be used to forecast and diagnose faults, guide normal running and preventive maintenance of equipment.