Abstract: The reason for fracture of high-strength 35CrMo steel bolt during necking process was analyzed by the methods such as chemical composition analysis, microstructure and fracture observation and mechanical property testing. The results show that the fracture of 35CrMo steel bolt had a typical brittle fracture characteristic. The microstructue of the 35CrMo steel was non-uniform, resulting in the relatively low tensile strength. During the straightening of necking process, when the pulling force at the necking part of the bolt was relatively high, the bolt fractured.