Abstract: The flat round head square neck bolt of 304 stainless steel fractured during the second fastening process. The causes of bolt fracture were analyzed by means of macroscopic observation, chemical composition analysis, fracture analysis, metallographic examination, metal streamline detection, coloring penetration test and mechanical property test. The results show that there were no abnormalities in the microstructure,non－metallic inclusions,metal streamlines and internal cracks of the bolt samples, and the chemical composition analysis and tensile strength test results met the requirements of the corresponding standards. The fracture morphology showed obvious cleavage fracture characteristics. The fracture property of the bolt was brittle overload fracture, and the fracture reason was that there was stress concentration in the initial area of the head fracture.