Aiming at the problem of open circuit fault diagnosis of direct drive permanent magnet wind power converter, a fault diagnosis method based on generator side current distortion law is proposed. This method studies the distortion law of three-phase current under different open circuit fault types, summarizes the distortion law and designs diagnostic variables, monitors the three-phase current of generator side after low-pass filtering in real time and calculates diagnostic variables, then obtains diagnostic signals according to the diagnostic variables, and finally completes real-time fault diagnosis according to the diagnostic signals, On the basis of single tube fault diagnosis method, a method supporting double tube fault diagnosis is proposed. The experimental results show that this method can diagnose single tube fault and double tube fault accurately and quickly, with low cost, good robustness and less misdiagnosis and missed diagnosis.