A 2-DOF dynamical model of nonlinear rear suspension system composed of main and auxiliary spring of heavy truck was created. The RMS amplitude of vertical acceleration was figured out by Matalab/Simulink and the optimizing model was developed. The aiming function was the arithmetic average of RMS amplitude of acceleration under empty load, critical load and full load. Suspension construction parameters, stiffness ratio and critical load ratio were optimized by dynamics. The example showed that compared with methods of proportional middle term and average load, the smoothness of truck with optimum stiffness ratio and critical load ratio has been improved.