Abstract: A dynamic model for a four-axle power car was established and analyzed by using software SIMPACK and the nonlinear response analysis in time domain with random track irregularity to improve its dynamic performances. The numerical results show that the optimum damping coefficients of the secondary lateral damper, the primary and secondaryvertical dampers are often certain forthis kind of power cars, but the effect of the change in unloading force on the riding quality can be reduced by increasing unloading velocity to ensure its riding quality. For a four-axle power carwith secondary big deflection helical springs, it is important that the secondary lateral stop clearance matches well with the secondary lateral damping characteristic to obtain a good dynamic curve negotiating performance.