Abstract: This study combined with production practice and vibration test conditions.The resutls of cracking in vibration test of 30CrMnSiA steel self-locking nut were comprehensively analyzed from macroscopic crack,fracture morphology,microstructure and microhardness.The result show that when the operator changes the height of the positioning pin,a step is formed at the straight edge of the nut,resulting in stress concentration and a crack source is formed at the bottom of the inner thread,and then fatigue cracks are generated through vibration test.Choosing the right height of the positioning pin and adopting the correct processing technology can effectively prevent such problems.