Abstract: Rainfall is the main environmental factor for the failure of slopes. To study the effect of the rainfall on the slope stability, the most important thing is to obtain the seepage field in slopes under rainfall condition. By using the Fourier integral transform, the analytical solutions for water infiltration into the slopes under rainfall, of which the intensity is less or more than saturated permeability coefficient of soils, are developed, and the unified analytical expression for water infiltration into the slopes under the two rainfall conditions is presented. The analytical solution can consider the dynamic changing process, in which the boundary condition of slopes changes from flux boundary to pressure one, as well as the properties of unsaturated soils and the effect of slopes. It is convenient to study the laws of water infiltration into slope more comprehensively. By comparing the analytical solutions with those obtained from numerical simulation, it is proved that the analytical solutions are reasonable, simple and efficient in computation. They can be used to calculate the ponding time and the pore water pressure at arbitrary point of the slope at any time, as well as to evaluate the effect of the rainfall infiltration on the stability of unsaturated soil slopes.