Abstract: When the shearer cuts the coal seam, the planetary gear transmission system of the traction unit is in a variable speed working condition, and the structure of the planetary gear transmission system is complicated. This leads to a variety of modulation phenomena of vibration signals, which affects the extraction of fault characteristics of the planetary gear transmission system. In order to solve the above problems, a fault diagnosis method of shearer planetary gear transmission system based on envelope order spectrum analysis is proposed by combining order analysis technique with envelope spectrum. Firstly, the method performs Hilbert envelope analysis on the non-stationary time-domain vibration signal of the planetary gear transmission system. Secondly, the method performs computed order tracking on the non-stationary time-domain envelope signal, converts it into equal angle angular domain signal. Finally, the method performs the fast Fourier transform on the equal angle angular domain signal to obtain the envelope order spectrum of the signal. The simulation analysis of this method and the fault diagnosis test of planetary gear transmission system under variable speed conditions based on the comprehensive test bed for power transmission fault diagnosis are carried out. The results show that it is difficult for the order analysis to extract the characteristic order for the vibration modulation signal under variable speed conditions. However, the envelope order spectrum analysis method can extract the characteristic order effectively and realize the accurate fault diagnosis of planetary gear transmission system.