This paper investigated the nonlinear dynamic behavior of seven degreeoffreedom automobile with suspension system of sigmoid magnetorheological damper stimulated by speed control hump(SCH). Based on the Lagrange equations, the twoorder differential equations were developed to describe the motion of automobile, and the fourthorder RungeKutta method was used for the numerical simulation. The bifurcation was analyzed by taking the height of SCH as bifurcation parameter, and the movements of the automobile at some values of height were analyzed by using time series, phase portrait, Poincaré map. The height ranges of SCH were obtained when the chaos vibration appeared, and the route to chaos was analyzed.