Abstract: Once a driver loses control of an operating vehicle on the highway,the guard rail installed as a protection will be important to eliminate the risk of fatal accidents during offroad crashes and collisions with hazardous roadside objects. The W-beam guard rail is one of the widely used passive safety devices designed to absorb loads applied by impacting vehicles. It can absorb the impact energy from large vehicles through the deformation of beams,blockouts and posts according to the traditional design method. However,contributions from the soils which the posts are rammed into are neglected. To this purpose,two computing models with and without soil bodies were considered,respectively. The trajectory of the truck,the deformation of the guard rail and the soil were observed during crash simulations. Moreover,the energy-absorbed ratios of different components in the guard rail system were also analyzed. A comparison between the two models indicates that the soil bodies adjacent to the post beneath the ground may become instable under shock loading,and more than 10% of the total impact energy is dissipated by the soil bodies. Consequently,the general fixed-base model cannot describe the real situation accurately enough.