Abstract: In landfill slopes composed of a mixture ofsa coarse and fine particles, rainfall infiltration would cause the migration of fine particles and thus change the soil hydraulic properties affecting slope stability. To investigate the intrinsic seepage-erosion coupling evolution of landfill slopes, a coupled model of unsaturated seepage and internal erosion was established based on the theory of continuum mechanics. This model was implanted into the COMSOL Multiphysics finite element to simulate the seepage-erosion coupling behavior of unsaturated landfill slopes under four different rainfall conditions. The coupled response law of the unsaturated landfill slopes was then analyzed in conjunction with the infinite slope model. The results of numerical simulation show that, based on the assumption of five-phase mixture in porous medium, the equations of porosity evolution, erodible fine particles content, and liquefied fine particles concentration derived from the continuum mechanics can effectively capture the changes in permeability and stability of soil caused by the erosion and migration of fine particles by seepage. Using the COMSOL Multiphysics numerical simulation platform, the partial differential equation (PDE) modeling with coefficients was used to simulate the multi-field, multi-phase coupling process of seepage, stress, porosity change, erodible fines content, and liquefied fines concentration. The soil porosity and liquefied fine particle concentration increase with the advance of the wetting front and internal erosion, while the content of erodible fine particles decreases. Internal erosion mainly occurs at the location of the wetting front, and the degree of hazard to the slope of the four rainfall conditions is in the following order of: homogeneous, decreasing, single-peaked, and increasing.