Abstract: A bolt for high-pressure vessel fractured in process of tightening before the hydrostatic test preparation, and through chemical compositions analysis, mechanical properties testing, metallographic examination and fracture analysis of the fractured bolt, the fracture reasons of the bolt were found. The results show that there were more inclusions in material of the bolt, and the presence of inclusions became crack initiation at the grain boundaries. The cracks expanded and hollowed under the force of the bolts particularly in its subsequent processing (tensile stress and heat treatment internal stress), and consequently a smaller force applied to the bolt in the tightening process could lead to the fracture.