Abstract: ［Objective］ The fracture of seat back bolts in high-speed railway EMU (electric multiple units) trains results in the back support performance failure and the angle adjustment feature malfunction. A high frequency of bolt fractures with consistent fracture pattern in seatbacks is found in maintenance inspections. Thus, it is necessary to identify the causes of the bolt fractures and propose corresponding countermeasures. ［Method］ Chemical composition checks, tensile tests and mechanical performance tests such as shear and hardness tests are conducted on the bolts, to evaluate the mechanical properties and verify the bolt quality. Metallographic analysis and electron microscopy scanning are employed to examine the macro and micro characteristics of the bolt fracture surfaces, identifying the types of bolt fracture. Theoretical analysis of the service conditions of the bolts is finally conducted to determine the primary cause of bolt fractures. Structural modifications are made, including the addition of auxiliary support brackets, to improve the loading conditions of bolts. ［Result & Conclusion］ The bolt fractures are not due to inherent quality issues but are identified as ductile fatigue fractures. The bolts are subjected to asymmetric loads under cantilever structures, and the resulted bending will lead to reduced fatigue performance and subsequent fractures. After implementing the countermeasures, no similar bolt fracture issues occurred during one year of high-speed railway EMU operation.