Abstract: The rear axle differential mechanism gear of a Pickup failed in the impact test of the whole vehicle transmission system. By chemical composition analysis, metallographic examination and fracture analysis of the gear, it is found that the failure of the gear has nothing to do with the defects in the material and the quality problem of the base metal. A finite element model calibrated with measured data was established to simulate the stress of the gear under the failure condition, and the fundamental cause of the gear failure was determined. The results show that the insufficient strength of the gear tooth is the fundamental cause of the gear failure. After design optimization, CAE was used to analyze the optimized scheme and the scheme was passed. Then bench experiment was used to verify this new scheme and whole vehicle verification was also carried out to solve the problem. At the same time, suggestions were put forward to improve the gear strength. The present work can provide reference for preventing the occurrence of similar faults.