Abstract: The ride comfort and road friendliness are 2 important indicators that reflect the vehicle suspension performance. In order to improve the road friendliness of heavy-duty vehicles,based on a heavy-duty vehicle dynamics model with seven degrees of freedom,this paper builds a kinematic equations of semi-active suspension system and designs a semi-active optimal suspension controller.Aiming at the ride comfort and road friendliness of vehicles,an optimal semi-active control method for vehicle suspen-sion is put forward by taking the road surface roughness into consideration.A detailed mathematical formulation is also provided. The simulation results show that when running at the speed of 20 m/s on a road with a surface of the C level,the effective root mean square of vertical acceleration of the vehicle body reduces by 3.42% and of the driver's cabinet reduces by 46.4%;tire dam-age on the road decreases by 2.10%.The semi-active suspension control can effectively maintain the ride comfort,while reduce the dynamical force of vehicles against the road surface.Compared with the traditional suspension of passive control suspension,it can improve the suspension performance.