Abstract: Suffusion in gap-graded soil involves selective erosion of fine particles through the pores formed by coarse particles under seepage forces. As the fines content (FC) decreases, the hydraulic and mechanical behavior of the soil will change, posing a huge threat to engineering safety. In this study, we first conduct a series of experimental tests of suffusion by using gap-graded soils and then analyze the evolution process of suffusion and the effect of the hydraulic gradient. Subsequently, according to the physical model, a discrete element method (DEM) numerical model with dynamic fluid mesh (DFM) is developed to extend the experimental study to the pore scale. Our results reveal the migration process of fines and the formation of erosion zones. A parametric study is then conducted to investigate the effect of the hydraulic gradient, fines content, and K0 pressure on eroded weight. The results show that the eroded weight increases with the increase of the hydraulic gradient and fines content but decreases with the increase of K0 pressure.