Taking K6 wheelset of railway freight car bogie as the main research object, the Simpack multi-body dynamics model was established. By setting the corresponding wheel diameter difference, the change of wheel/rail vertical force, transverse force and lateral displacement with the change of wheel diameter difference was emphatically analyzed, and the corresponding relationship of wheel/rail force corresponding to different wheel diameter difference was analyzed. Analysis results show when the wheel diameter difference appears, wheel vertical force and lateral force on the contact surfaces have the corresponding difference, and the difference increases with the increase of wheel radius. The lateral displacement of the wheelset with wheel diameter difference also increases with the increase of wheel diameter difference, and within a certain range, the variation difference is basically linear with the wheel diameter difference.