Abstract: Soil-rock mixtures are composed of coarse particles and fine particles. The fine particles are under the action of seepage flow, resulting in suffusion. The existing suffusion models are unable to quantitatively analyze the evolution of each grain group during the suffusion process of soil-rock mixtures and cannot comprehensively understand the changes in soil parameters during the suffusion process of soil-rock mixtures.Therefore, in this paper, through the analysis of the force balance of particles, the critical conditions for the migration of soil particles and the mass of the migrating particles are deduced, and a method for quantitatively calculating the soil gradation at any time during the suffusion process is obtained. Moreover, by coupling the seepage flow control equation and the porosity equation, a suffusion model that can quantitatively analyze the mass changes of each grain group of soil-rock mixtures is established. The feasibility and accuracy of the established model are verified in combination with experimental data. Through the established model, the changes in the mass of each grain group of soil-rock mixtures during the suffusion process can be predicted, and furthermore, the changes in the dry density and porosity of the soil mass can be obtained. This research realizes the quantitative analysis of each grain group during the suffusion process of soil-rock mixtures and further reveals the suffusion mechanism of soil-rock mixtures.