The open circuit fault of the direct-drive wind power system converter causes problems such as half-phase loss of AC current on the machine side, voltage fluctuation on the DC side, fluctuation of generator speed, etc., which seriously affects the operational stability of the wind power generation system. A fault diagnosis method for open-circuit fault of permanent magnet direct-drive wind power system based on real-time current amplitude is proposed. The real-time current amplitude is calculated by using the running characteristics of the motor, so that the fault diagnosis algorithm can diagnose and locate the fault in real time. Comparing the current amplitude in the fault state with the set current amplitude threshold eliminates the misdiagnosis caused by errors. The position of the abnormality of the current amplitude is used to determine the position of the fault switch tube. Experimental results verify the correctness of the theoretical analysis and the effectiveness of the fault diagnosis method. Compared with other methods, the detection speed of the method and the diagnostic capability of multiple switch faults are verified.