Wireless power transmission (WPT) technology shows great potential for application in electric vehicles and other fields, but its high-frequency, high-voltage operating characteristics and complex operating conditions can easily lead to the aging and failure of power switching devices, seriously threatening system reliability. To address this issue, this paper proposes a fault diagnosis method based on current paths. First, the system's current paths under normal and open-circuit fault conditions are analyzed. Then, a DC-side voltage model is established, and by analyzing the absence of voltage characteristics and the timing information of switch device trigger signals, the faulty device can be precisely located. Finally, an LTspice simulation model is constructed to validate the accuracy of this method, providing a theoretical foundation for research on fault diagnosis in wireless power transmission system.