Abstract: Surface cracking of 42CrMo steel turbo worm was found during assembly. The causes of worm cracking were analyzed by means of macro analysis, chemical composition analysis, quenching surface residual stress test, micro analysis, metallographic examination, energy spectrum analysis and hardness test. The results show that the surface crack of the 42CrMo steel turbo worm was quenching stress crack. The high mass fraction of manganese in worm material and the superposition of thermal stress and structure stress during quenching process caused the worm to cracking along the axis direction.