Abstract: A grade 12. 9 high strength hexagon socket bolt was used to fix the lathe tool, which fractured in service. The causes of fracture were analyzed by means of macroscopic observation, fracture analysis and chemical composition analysis, combined with bolt installation position and working environment. The results show that the cleaning fluid entered from the gap between cutter head and tool holder during tool cleaning, which resulted in corrosion and stress corrosion fracture occurred under the working stress.