Abstract: The front supporting rod of the main reductor was fractured during fatigue test. The fracture reason was analyzed by means of fracture analysis, chemical composition analysis, hardness testing and metallographic examination. The results showed that unsatisfactory weld figuration and stomata defect induced by the improper control of welding process were the main reason, also the part severe overheat and grain coarsening were caused at the local weld, which decreased the fatigue strength of the local weld.