Abstract: Abstract: The spline shaft of a domestic extended range electric vehicle generator, made of SCM440H material, broke while the vehicle was running for 55000Km. To investigate the cause of its failure, macroscopic morphology, surface morphology, metallographic structure, and chemical composition analysis of the failed component were carried out through scanning electron microscopy (SEM), metallographic microscopy (OM), and direct reading spectroscopy (OES). The results indicate that the reason for the failure of the keyway shaft of the generator is the stress concentration at the R-angle of the keyway, which leads to cracking and the formation of fatigue sources at the cracking site, ultimately resulting in multi-source fatigue fracture.