To deal with the problem of the serious fretting wear of a powertrain gear shaft shoulder in working process, the finite element model of the gear shaft was built with the finite element software ABAQUS. The required minimum interference was calculated through the calculation of interference fit between the gear shaft and the bearing. Simulation results were analyzed to study the effects of the interference fit on the fretting wear of the gear shaft shoulder. By analyzing the effects of the different interferences on the force and deformation of the gear shoulder, the conclusions are as follows: the force between the bearing inner ring and the shaft shoulder increases correspondingly to the increase of the interference, but on the contrary, the relative deformation (i.e. displacement amplitude) between the bearing and the shaft shoulder contact surface decreases correspondingly to the increase of the interference. Therefore, it not only guarantees that there is no gear shaft material plastic deformation in the stationary state, but also enables the gear shaft to have the sufficient contact pressure to transmit the effective torque when the interference between the gear shaft and the bearing increases to the appropriate value, which can effectively reduce the fretting wear of the gear shaft shoulder.