Abstract: During the bolt assembly and screwing process of installing the four-point contact ball slewing bearing for a certain type of radar azimuth drive system, one high-strength bolt of grade 10.9 fractured. To study the inherent mechanism of this problem, the failure analysis was carried out on the failed bolt. The chemical composition, metallographic examination, and hardness test of the bolt all showed no abnormality. The scanning electron microscopy of the fracture surface revealed that the central source area, the propagation area, and the edge transient rupture area exhibited axial dimples, radial propagation, and dimple characteristics respectively, indicating an overload fracture under tensile loading. Through the pre-tightening stress analysis, it is believed that differences in surface treatment methods and lubrication conditions can lead to differences in friction coefficients, and the secondary screwing of the electric torque wrench can generate a larger tightening torque. This problem can be solved by adjusting the bolt tightening torque and fastening it in a three-step method according to a star-shaped sequence.