The brush and slip ring system is an important part of doubly-fed induction generator excitation system, but the poor contact of brush and ring caused by slip ring surface damage in operation influences the excitation stability. For the problem, this paper proposes a new method of slip ring surface damage fault diagnosis based on the vibration signal. Firstly, the vibration method principle, route of transmission and failure mechanism of the slip ring are analyzed. And then the experiment platform of the brush and slip ring simulation is set up for real-time vibration signals before and after fault operation, and the wavelet energy spectrum method is applied to fault diagnosis of slip ring interface. The abnormal vibration is found out by comparing and analyzing the wavelet energy spectrum distribution and the proportion of vibration signal before and after the fault, which provides the basis for the fault diagnosis of the slip ring contact surface. This work is supported by National Natural Science Foundation of China (No.51577050).