In allusion to the open-circuit fault in the insulated gate bipolar transistor (IGBT) of the three-level inverter,a fault diagnosis method of measuring the voltage on both ends across the diode bridge wall as fault information to analyze each fault condition is proposed by studying on multiple fault form of single bridge wall .Then the fault signal can be converted to feature vector with wavelet transform theory according to the features of the fault signal and the wavelet function ,and support vector machine(SVM) is applied to fault diagnosis. In order to improve the diagnostic accuracy, the improved cuckoo optimization algorithm is used to optimize parameters, so as to get the optimal model to realize the multiple fault diagnosis classification. By the simulation of diode clamp three-level inverter , the accuracy diagnostic results show the feasibility of this method.