Abstract: In order to clarify the cause of the fracture of the connecting bolts of the pitch bearing of a wind turbine generator unit in a wind farm of Huaneng Group, a failure analysis was conducted on the broken bolts. Through chemical composition analysis, metallographic structure analysis, geometric dimension measurement, mechanical properties, and fracture observation, it was found that the main reason for bolt fracture is due to the small radius of the tooth base at the first screw tooth. At the same time, the large force at the first screw tooth has a significant promoting effect on the initiation and propagation of cracks. The reasons for bolt breakage identified in this article provide important technical support for ensuring the safe and stable operation of wind turbines.