In order to get a better convergence and transition for passive bipedal robot research,this article analyzed the initial model of passive biped robot,i.e.the two-dimensional rimless-wheel model.Its dynamic model was constructed and its motion characteristic was analyzed with Adams.The effect of initial velocity,quality,foot radius,the number of spokes,the length of spoke,moment of inertia and slope gradient on the movement of rimless-wheel was analyzed.Results show that a right initial velocity and slope gradient,larger quality and foot radius,more and shorter spokes,as well as larger moment of inertia can lead to a fast and smooth cycle motion for rimless-wheel.