Abstract: The spline shaft of the generator of a domestic extended-range new energy vehicle is made of SCM440H alloy steel and breaks when the vehicle runs for 55000km. To explore the failure cause, the macro morphology observation of the fracture surface of the failed part, the fracture surface analysis, the metallographic microstructure analysis, the chemical composition analysis and the hardness test are carried out successively by using a stereomicroscope, a scanning electron microscope（SEM）, a metallographic microscope（OM）, an optical emission spectrometer（OES） and a hardness tester. The results show that the main cause of the failure of the spline shaft of the generator is that the depth of the effective hardened layer at the involute is lower than the technical requirements,which reduces its ability to resist stress deformation. At the same time, there is stress concentration at the R corner of the keyway. Under the combined action, cracking occurs at the R corner of the keyway and a fatigue source is formed at the cracked part, ultimately leading to multi-source fatigue fracture.