Abstract: Metro axle box spring system is a kind of complex bolt connection structure. The mechanism of lower cover bolt fracture occurred during application is studied by using finite element method (FEM). Due to the insufficient antisliding ability of the original structure Utype primer adjustment pad, the sliding contact occurs under the action of longitudinal load, and the bolt is subjected to large alternating bending moment and eventually breaks. Modified round milled sand adjustment pad could significantly improve the antisliding ability and the bolt loading environment, reduce the bending moment load on the bolt. The results show that the contact state of the fastened structure directly affects the life span of the tightening bolt.