The output pinion of a MW-class wind turbine yaw gearbox has been broken down after about3 years in the grid. The comprehensive analysis is carried out on the failure gears,the causes of gear damage,such as tooth fracture,design factor,production factor,factors of assembly and operation,system protection factor,etc. are analyzed one by one,the final positioning between the thrust pad bearing and yaw ring interference is the direct cause. Bladed software is used to simulate the impeller imbalance and verify the wind field.It is concluded that the deviation of the installation angle of a blade of the wind turbine causes the aerodynamic imbalance of the wind wheel is the root cause of the failure.