Planetary gear transmission is shaft gear train, at the same time, the number of teeth meshing is a lot and the meshing position is dynamic, The vibration response is more complex than the traditional fixed axis gear transmission, which is not conducive to the extraction and application of fault diagnosis characteristics. In order to study the fault feature and its variation law, taking the local fault feature extraction of planetary gear as an example, based on the theoretical analysis of the motion characteristic frequency, the simulation experiment platform is set up, by using the method of that amplitude demodulation of Hilbert transform for structural vibration shock, we analyzed the local fault frequency characteristics and the variation of star wheel under the different speed conditions. The results show that the method can get the characteristic frequency and frequency doubling of the planetary fault, and the fault characteristic frequency can also be used, for example, it can be a reference for fault diagnosis of wind power generator and other equipment.