Abstract: ［Objective］ Bogies of urban rail transit vehicles often adopt a swing-arm positioning structure, within which the primary steel springs are prone to fracture, and typical fractures occur at the first working coil in contact with the support ring. Various factors contribute to the fatigue fracture of steel springs, necessitating an in-depth analysis. ［Method］ Considering actual vehicle operating conditions, the stress calculation method for steel springs is optimized. Using specific project design parameters as inputs, the stress levels of the steel springs under lateral load conditions are evaluated, and the proportion of stress contributions is calculated. A bench test is conducted on the primary steel spring set of a selected project bogie, and the setup and tooling of the bench test, including the arrangement of the inner and outer strain gauges on primary helical steel springs, test loading procedures, and data analysis methods, are expounded. Stress test results for the inner coil of Spring Group B are analyzed. Using a measurement point on the swing-arm side as an example, the operating conditions of steel springs when vehicle adopts a swing-arm positioning structure is analyzed, with emphasis on the impact of swing-arm rotation on stress at different steel spring positions. Finally, countermeasures against spring fatigue fractures are proposed by improving spring fatigue stress and controlling catenary wire continuity. ［Result & Conclusion］ The fatigue fractures of such springs are not caused by insufficient design margins, but rather by manufacturing defects, such as sand inclusions, powder aggregation, and improper manual grinding, that result in discontinuities in catenary wires, leading to stress concentration in the steel springs.