The connecting bolt of a blowout preventer oil circuit filter SCM435 broke during use, causing the filter to fail to work normally. In order to find out the failure causes of connecting bolts, the microstructure, chemical composition, hardness, mechanical properties and fracture morphology of fractured and unfractured bolts were studied by means of metallographic microscope, tensile testing machine, impact testing machine, microhardness tester, scanning electron microscope and energy dispersive spectrometer. The results show that the microstructure of fractured and unbroken bolts are tempered cordite and ferrite, the fractured bolts have more inclusions, high carbon content, and the hardness and mechanical properties of the fractured bolts are lower than those of unbroken bolts. The fracture bolt fracture is in the two sections of parallel and oblique 45° angle, manifested as a mixed fracture form of ductile fracture and brittle fracture, the fracture source originates from the inside. It can be seen that the fracture bolt is due to the uneven distribution of brittle phases such as internal carbides. Under the action of continuous external force, the crack source is preferentially formed, and the bolt finally breaks with the expansion of the crack.