The wheel dynamic model is established based on FEM and modal test for the monobloc steel wheel with S-web. The cross section deformation of 60kg/m rail and the effect of structure under rail are also considered to obtain the rail dynamic model. The wheel-rail contact system is analyzed in the different contact position and wheel’s modal damping ratio. It is shown that the wheel mode with zero nodal circle has the most obvious effect on the stability of contact system. The increment of lateral creepage can lead the contact system to become unstable, and the effect of creepage is more obvious in flange contact than tread contact. The effect of contact position depends on the yaw angle and the friction coefficient in contact area. The increment of wheel damping can decrease the number of unstable frequency, and the effect of damping is more obvious in flange contact under large yaw angle.