According to the cracks in the gearbox of a certain type of diesel locomotive,the finite element model of gear box was established to make the life prediction by using the random vibration fatigue theory and the linear fatigue cumulative damage theory of Palmgren-Miner. The life prediction of the original structure differs from the actual statistical results by only 26 000 km,and two optimized structures are proposed based on the finite element results of the original structure. The predicted fatigue life of the two optimized structures is higher than that of the original structure by 1. 032 million km and 1. 763 million km respectively.After modal and excitation frequency analysis,it is concluded that transverse excitation is the main cause of structural resonance.By comparing the modal frequency and excitation frequency of the three structures,it is found that the lower the excitation frequency energy value corresponding to the modal frequency is,the longer the fatigue life is. Therefore,the frequency corresponding to the high excitation energy should be avoided when optimizing the structure of the gearbox.