Abstract: ​: This paper systematically investigates the fatigue fracture mechanism of an eccentric bolt at the connection between the rear upper control arm and the brake of a test vehicle in an automotive OEM. Through macroscopic fracture analysis, microscopic fracture observation, chemical composition detection, hardness testing, and metallographic structure analysis, the study reveals that: The bolt material (SCM435 steel) and heat treatment state met standard requirements. However, improper torque application during wheel alignment caused bolt loosening. Under cyclic loading, stress concentration at the thread root initiated crack nucleation and propagation, ultimately leading to fatigue fracture. Based on these findings, improvement measures were proposed, including optimized assembly processes (high-precision torque control, residual torque monitoring) and bolt design modifications (increased transition fillet radius, optimized thread structure). Subsequent validation tests confirmed the elimination of similar failures. This research provides practical engineering references for preventing fatigue failures in analogous metal components.