Abstract: The fracture reason of pin shaft in a movable mould frame was analysed by means of fracture analysis, chemical composition analysis, non-metallic inclusions detection, mechanical properties testing, metallographic examination, force and welding process analysis. The results showed that the coarse martensite or crack caused by improper welding process or unreasonable structure design of welded joint were the main reason of the brittle fracture on the shaft. Meanwhile, the preventive measures were recommended.