Due to rainfall, the soil-rock differential weathering interface of spherical weathered granite soil slopes is prone to evolve into a preferential seepage channel and undergo seepage-induced suffosion, which accelerates the deformation and instability of these slopes. However, little research has been carried out on the characteristics of seepage-induced suffosion and the migration of fine particles. Based on the unsaturated seepage theory of porous media, a numerical calculation framework is established to accurately describe the seepage-induced suffosion process at the soil-rock interface, considering the coupling relationship between the fine particle migration, suffosion initiation response and unsaturated seepage. The finite element method is used to construct a seepage-induced suffosion model for unsaturated granite residual soil under the effect of preferential flow. Based on the seepage-induced suffosion process of homogeneous soil columns, the suffosion characteristics of preferential flow under three typical soil-rock interface burial states are systematically investigated. The results show that the soil-rock interface and the matrix permeability of spherical weathered granite soil slopes are highly variable, with the wetting front forming a downward depression infiltration funnel, and the degree of depression of the wetting front becomes more pronounced as rainfall continues. The degree of fine particle loss is related to the burial state of the soil-rock interface, in which the preferential flow potential suffosion of the under-filled soil condition is the most significant, and even excess pore water pressure occurs at the interface, which is the most unfavorable to the stability of this type of slope. The research results can provide a scientific basis for accurately evaluating the stability of spherical weathered granite soil slopes under rainfall conditions.