The failure analysis of the planetary gear used in the vertical mill is critical to help improve the life cycle of the equipment. In this paper, the failure analysis of the planetary gear used in the vertical mill planetary reducer was conducted. The failure mechanism of the planetary gear was figured out by means of macroscopic examination, microscopic analysis and finite element analysis. The results show that contact fatigue is the main reason that cause the fracture of the planetary gear. The influence of the wall thickness of the planetary gear on the stress of the gear tooth was also analyzed, and the causes of the fatigue micro cracks of the planetary gear was verified. The prevention methods are then proposed to improve the design and reliability of the planetary gear.