Abstract: A partial failure vibration signal model of spur gear was established starting from gear drive mechanism and considering both amplitude modulation and frequency modulation effects of meshing vibration due to partial spur gear damage. Based on partial failure model of spur gear, structure feature of planetary gear and vibration signal generation and changing rule were analyzed, and partial failure vibration signal model of planetary gear was established. Load test of rocker arm with different gear partial failures and spectrum analysis of collected vibration signal were conducted, the results show that the model of gear fault vibration signal model can objectively and accurately reflect failure state of rocker gear, and using the method of spectrum analysis can achieve partial failure diagnosis of rocker gear.