Abstract: A method based on S transform and support vector machines(SVM) is presented for fault diagnosis of power electronics circuit,in which the S transform time-frequency analysis on fault signal is used to extract the features corresponding to various faults,then fault types are identified through the pattern recognition classifier based on SVM.The simulation result of fault diagnosis of thyristor in three-phase full-bridge controlled rectifier circuit shows that the proposed method can accurately diagnose fault and locate fault element for power electronics circuit,it has excellent robustness to noise,and it just needs small number of training sample,thus it has very good practical value and applicable prospect in the solution to fault question for power electronics circuit.