A fault diagnosis method based on wavelet packet transform and support vector machine (SVM) is proposed to solve the problem of complex calculation and low accuracy in the open-circuit fault diagnosis of neutral point clamped (NPC) three-level inverter. The wavelet packet transform is used to decompose the fault signals with their timefrequency characteristics into several frequency bands, and the wavelet packet energy is extracted as the fault feature vector. The fault feature vector is taken as the input of SVM, the radial basis kernel function is used for cross-validation, and multiple SVM models are obtained through training the data set. Finally, the randomly selected test set is input into the trained model. Simulation results show that the method can diagnose the IGBT open-circuit fault and has higher fault recognition accuracy.