Abstract: The fracture phenomenon of 12.9 grade high strength bolt for a coal mill occurred. The reasons of the fracture bolt was analyzed by means of macroscopic observation, scanning electron microscope analysis, chemical composition analysis, metallographic examination and mechanical property test. The results show that the fracture property of the high strength bolt was line source fatigue fracture, and the fatigue source was located at the stress concentration of the thread bottom. The tempering temperature of high strength bolts during quenching and tempering was high, which made the strength of bolts lower than the standard requirements, and finally led to the fracture of bolts.