Fault diagnosis is the premise to realize fault tolerant control, in order to improve the reliability of the motor system, and for the open circuit fault of the inverter power tube in brushless dc motor drive system, the wavelet transform is used to analyze the three phase current signal of the motor. The fault point of the signal is detected accurately according to the product of 3 layers of wavelet transform detail signals, the energy of the low frequency wavelet signal is regard as the extraction feature to identify the phase sequence of fault inverter. This method has the features of high reliability and a simple algorithm for extracting the characteristic signal. The simulation results verify the effectiveness and accuracy of the proposed method.