Abstract: The blade connecting bolt of a wind turbine fractured during torque tightening maintenance. The fracture causes of the connecting bolt were analyzed by means of macroscopic analysis, metallographic examination, chemical composition analysis and mechanical property test. The results show that the metallurgical quality of the connecting bolt was poor, there were more nonmetallic inclusions and the crack source was formed in here. Under the action of periodic alternating load, the crack source expanded continuously without being found, and the remaining working cross section of bolt became smaller. Finally, during the torque tightening maintenance, the fracture occurred at the root of the first thread where the bolt and nut were engaged.