Using the Richards’ equation of saturated-unsaturated seepage, the commercial Multiphysics finite element software COMSOL is adopted to deduce the governing equations of two boundary conditions with the pore water pressure as the control variable for the infiltration and seepage (overflow) boundary conditions in the rainfall infiltration problem of slope. Based on the two-dimensional soil column model and the published models, the value of the boundary coupling length scale L in the governing equation is discussed, and L equal to 0.001 m is found reasonable. A simple two-dimensional slope model is then established, and the governing equations of the above boundary conditions are applied to analyze the infiltration and seepage law of rainfall with different intensities (long and weak, short and strong). The results show that when the rainfall intensity is 4 mm /h, the actual infiltration rate is always equal to the rainfall intensity, and the water content of the surface soil has increased from 0.29 to 0.35. When rainfall lasts for 75 h, the surface seepage occurs at the foot of the slope, the total infiltration amount of the study area at 200 h is 39.068 m3 ; when the rainfall intensity is 40 mm /h, the actual infiltration rate first equals the rainfall intensity, and then gradually decreases, and the water content of the surface soil increases from 0.29 to 0.415 (saturated). When rainfall lasts for 4 h, the surface seepage occurs at the foot of the slope, the total infiltration of study area at 20 h is 26.908 m3 , which is far less than the former. This conclusion is consistent with the existing rainfall infiltration law in slope, which further proves the reliability of the above boundary condition governing equations that provides a feasible method for the boundary condition problem in finite element analysis of rainfall in slope.