Abstract: The fracture causes of 35CrMo steel high strength bolts in the tower foundation ring of a 2 MW wind turbine were studied by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination and fracture analysis. The results show that the carbon content on the surface of the fracture bolt was far below the standard requirement; there was an obvious decarburization layer on the thread surface, and the micro-hardness of the bolt surface did not meet the standard requirements.The serious decarburization on the root surface of the thread tooth led to the decrease of the surface hardness and mechanical properties of the bolt, and led to a large concentration of organizational stress at the root of the thread, which caused the initiation of microcracks in the grain boundary weakening zone of the decarburization layer at the root of the thread tooth at the stress concentration position. The excessive content of internal inclusions promoted the crack propagation, and finally led to the fatigue fracture of the bolt. It was suggested to strengthen the supervision and inspection of the performance and manufacturing quality of high strength bolt materials.