Abstract: Usig the basic theory of particle flow code (PFC),based on PFC2D procedure of the discrete unit method,it is established a model of soft soil particle flow to simulate the contact stress field and displacement field of the filled slope of a soft soil roadbed,as well as the effect of using gravel piles on reinforcing the slope. In the analysis,macroscopic continuous supposition of the traditional continuous medium model is overcome;it is studied and compared the change in the displacement field of the shope during slip before and after being strengthened by the gravel piles,as well as the effects of different pile diameter,space and length of the gravel piles. The results show that pile length has an obvious influence on the antislip capacity,i.e. the capacity of a gravel pile is improved evidently with its length increases. The gravel piles with different diameters have different capacities of antislip,i.e. with an increase in pile diameter antislip capacity is also raised. The influence of a pile space on the antislip capacity should be determined according to the optimization of a pile diameter,so as to use the co-action of pile and soil.