Abstract: During the flaw detection process, longitudinal cracks were found on the threaded end of a 20CrMnTiH steel automotive motor drive shaft. The reasons for the cracking of the motor drive shaft were analyzed using macroscopic observation, scanning electron microscopy analysis, metallographic examination, chemical composition analysis, hardness testing. The results show that the cracking property of the threaded end of the motor drive shaft was delayed cracking. Unreasonable anti-seepage technology caused surface carburization at the threaded end, forming a high hardness carburized martensitic structure, increasing the sensitivity of the material to delayed cracking, ultimately leading to cracking of the threaded end of the motor drive shaft.