Abstract: A 42CrMo steel flywheel bolt was broken during use. The causes of bolt fracture were analyzed by means of macroscopic observation, scanning electron microscopy and energy spectrum analysis, metallographic examination, chemical composition analysis and hardness test. The results show that in the production process of bolts, hydrogen atoms penetratd into the shallow surface layer of bolts. In the subsequent use, the combined action of this part of hydrogen atoms and tensile stress led to the phenomenon of hydrogen embrittlement delayed fracture of bolts.