The invention discloses a method for detecting the fault of rotating diodes, which is characterized in that whether the rotating diode is at fault or not is judged by detecting specific value of the effective values of a first, a second, a third, a fourth, a fifth and a sixth subharmonic and the effective value of a twenty second subharmonic of exciting winding current of an exciter; when the specific value of the sum of the effective values of the first, the second, the third, the fourth, the fifth and the sixth subharmonic and the effective value of the twenty second subharmonic is less than 0.1, the rotating diode is in normal operation, and when the specific value is more than 0.1, the rotating diode is at fault; when the specific value of the sum of the effective value of the first, the third and the fifth subharmonic and the effective value of the twenty second subharmonic is more than 50, the rotating diode has arm short fault, and when the specific value is less than 10 and more than 1, the rotating diode is in one phase open circuit, when the specific value is more than 10, the rotating diode is in multi-phase open circuit. The fault judging method of rotating diode of the invention provides accurate and suitable control strategy for a generator when the rotating diode is at fault, and ensures that excitation system and the generator is in normal and safe operation.