Aiming at the problem that it is difficult to accurately predict the bolt pre-tightening force in the nonlinear bonding stage during the bolt tightening process, a bolt nonlinear loading model based on the micro-convex body contact of the joint surface is proposed. The model considers the variable stiffness phenomenon caused by the extrusion of three joint surfaces in the bolted structure, which satisfies the continuous and smooth change between different contact states. It can better characterize the nonlinear loading stage and variable stiffness process of bolted connection, and is more in line with the actual assembly situation.