According to MOSFET in the LLC series resonant switching power supply which plays a key role but has a high failure rate, the failure modes of MOSFET and their influences on the circuit is analyzed based on the Cadence/OrCAD PSpice environment in this paper, and the important input and output between the functional module of the circuit is selected as a fault feature extraction point, after which, the fault feature set is established, a support vector machine algorithm based on K-CV optimization and a neutral network algorithm based on GA optimization is applied to fault identification. The diagnostic results indicate that data of the selected point is basically retain fault characteristics of MOSFET in the circuit, both algorithms have high accuracy of fault diagnosis. The diagnostic strategy proposed in this paper has the characteristics of practical operability and simple operation.