Aiming at the problem of high crack rate in key positions of cast traction seats for railway locomotives during long-term service, a study was conducted on the post-weld fatigue failure mechanism at key positions of cast traction seats based on fracture analysis. Firstly, a experimentally validated finite element model of the traction seat was established, and it was determined that there is a significant stress concentration area at the large arc of the base of the traction seat under standard traction braking loads. Then, a high cycle fatigue test was conducted on the traction seat based on the standard load spectrum, reproducing the initiation and propagation process of macroscopic cracks in the stress concentration area. Finally, fracture analysis, scanning electron microscopy (SEM) observation, metallographic examination, and energy dispersive spectroscopy (EDS) were per-formed on the fatigue crack failure area, indicating the presence of obvious welding repair marks and heat-treated microstructure in the crack area. The research results indicate that the fatigue cracking failure of the traction seat is the result of the combined effect of stress concentration at the base arc and defects introduced by welding repair. It is important to focus on controlling the casting quality at the large arc of the base and avoid welding repairs. The research can provide theoretical guidance for the safe and efficient operation of railway freight and the maintenance and repair of parts.