An angle trajectory monitoring technology is studied using fast Fourier transform (FFT) and applied to the fault feature extraction and online diagnosis of the rotating rectifier of aero generator. Firstly, the excitation current signal of the AC exciter of the generator is continuously collected, and the interval between two sampling points is kept constant. Secondly, FFT processing is performed on the data collected to obtain fault characteristic information, and this process is completed in the sampling intervals. The fault characteristic information obtained by each processing can form a continuous change trajectory, and the fault of the rotating rectifier can be monitored and diagnosed online through studying the trajectory. Finally, the proposed method is verified by experiments.