Abstract: The fracture causes of the bolt connected with spindle of a wind turbine were analyzed by macroscopic inspection, chemical composition analysis, metallographic examination, mechanical property test, fracture analysis and finite element analysis. The results show that the fracture mode of the spindle connecting bolt was hydrogen induced brittle fracture. Improper control of the pickling process before the surface treatment of the bolt caused a large amount of hydrogen to penetrate, which was the main cause for the hydrogen induced brittle fracture of the bolt.