Abstract: The fracture phenomenon of high strength bolt connecting blade and pitch bearing of a large onshore wind turbine generator system occurred. The causes of bolt fracture were analyzed by means of macroscopic observation, chemical composition analysis, scanning electron microscope analysis, metallographic examination and mechanical property test. The results show that the reason for the fracture of the connecting bolt was the fatigue fracture of the threaded tooth bottom area. The main reason for the initiation and propagation of fatigue cracks was that there was a large amount of widmanstatten structure in the bolt raw materials, which led to the decrease of the comprehensive mechanical properties of the matrix. Under the action of alternating load, the stress concentration area preferentially initiated fatigue cracks and gradually expanded, eventually causing bolt fracture.