Abstract: For low reliability of existing fault diagnosis method of power converter of switched reluctance motor, the paper proposed a new type of fault diagnosis method of power tube. The new method takes main circuit of asymmetric half-bridge power converter as study object, and two kinds of common faults of open circuit and short circuit of power tube as diagnosis objects. It determines fault by comparing estimate value and actual value of bus current, achieves identification of fault phase combining with winding current of fault phase, and determines fault point by interrupting test if necessary. The paper selects conditions which are difficult to achieve fault diagnosis of motor operation with no-load, low-speed and high-speed for experiment and analysis, and the results indicate that the proposed method can complete accurate determination of fault type and location of fault point.