Abstract: The magnetic blocking alarm occurs during the engine test process of a certain type of aero-engine reducer; then, it is found out through the decomposition inspection that the tooth surface of an active gear of the reducer is peeled off. After the detailed analysis, it is confirmed that the small tooth top circle of the faulty gear pair leads to stress concentration; in the early supertorsion state, the gear contact mark the underweight border extends at the root position, and there is an obvious local stress concentration, which causes tooth surface injury; and there is the nozzle blockage and insufficient lubrication. Under the combined action of the above factors, the pitting erosion occurs, and pitting is then further extended to flafoliation. In view of this fault, the corresponding improvement measures are provided and validated effectively through tests. This troubleshooting experience can provide a reference for the similar gear faults in the future.