Abstract: The axial longitudinal cracking of the shaft gear for a coal shearer reduction gear box took place during the storage period of the warehouse. The causes of shaft gear cracking were analyzed by means of macroscopic analysis, chemical composition analysis, mechanical property test, fracture analysis and metallographic examination, combined with the processing technology of the shaft gear. The results show that the shaft gear was a hydrogne-induced delay crack. The micro cracks produced in the stress concentration part of the non-martensite structure at the surface of the axial gear under the joint action of hydrogen and internal stress. At the same time, the follow-up tempering was not sufficient, and the micro cracks expanded continuously, which caused the longitudinal cracking of the shaft gear.