Abstract: During the test of an engine reducer, the tooth surface of the driving gear was found to have inverted triangular damage. Through macro and micro observation, metallographic analysis, hardness testing and simulation calculation analysis, the failure mode and cause of the gear tooth surface damage were analyzed. The results show that the inverted triangular damage of the tooth surface is contact fatigue spalling, and the spalling pit originated from the micro-pitting zone below. The smaller tooth profile modification amount and tooth tip chamfer of the driven gear led to greater local contact stress exceeding the contact fatigue limit of the material, which is the cause for the spalling. Through the improvement measures of increasing the tooth profile modification amount and standardizing the tooth tip chamfering process to improve the machining quality of the driven gear, the reducer passed the examination smoothly.