The gearbox structure is complex, so it is difficult to determine the root cause of the problem after abnormal sound appears. The method of sound pressure signal analysis is used to analyze the abnormal sound of gearbox. The spectrum analysis method is used as the data processing method. The sound pressure of a kind of abnormal sound of gearbox is collected. By analyzing its frequency characteristics and structural characteristics, it is judged that the gear is abnormal. The accuracy of the inference based on the result of sound pressure analysis is verified by the shape and position detection of the gear. The results show that through the frequency analysis of the sound pressure signal, the root cause of the gearbox problem can be accurately determined.