Abstract: When the gas turbine starter was returned to the plant for repair and decomposition inspection, it was found that the reducer input drive gear was damaged and the positioning section of the elastic shaft was seriously glued in one direction. Through macro and micro observation, microstructure examination, and stress calculation of gear in deflection, it is determined that the failure mode is tooth bending fatigue, which is related to the larger vibration and impact load. The clearance fit between the front spline of the elastic shaft and the internal spline of the dynamic turboshaft led to radial displacement and vibration increase of the turbine rotor. As a result, the contact stress and bending stress of the gear increased significantly, resulting in fatigue fracture of teeth.