Abstract: The problem that the wheel unroundness of urban rail transit vehicle causes damage to running parts has been widely recognized. The internal mechanism of wheel unroundness is analyzed, and a vertical model of single wheel and rail is established. Under the action of braking force and traction force, the change of the vertical positive pressure when the wheel circumference is out of shape, the relationship between the vertical positive pressure and the longitudinal creep force of the wheel circumference are analyzed. The stiffness and damping parameters under the rail, and the effect of vehicle primary vertical suspension parameters on wheel unroundness are obtained. Research shows that compared with vehicle parameters, under-rail parameters have a greater impact on the development of wheel unroundness and play a leading role. When the parameters under the rail tend to be rigid, abnormal wear of the wheel will be restrained and the wheel will be round and smooth. When the parameters under the rail tend to be elastic, the unroundness of the wheels will be further aggravated and accelerated.