Based on the theory of continuous medium

a saturated-unsaturated seepage mathematical model is presented. The method of calculating unsaturated seepage field by FEM and the mechanism of rainfall infiltration and the rainfall simulation are proposed. Unsaturated seepage program SUSC is compiled based on above theory. Through the analysis of seepage field in a slope under rainfall infiltration

results show that the deeper as well as surface of slope are saturated rapidly due to rainfall

and surface water infiltrated to the deeper under slope. Pressure water head changes from high (zero) to low

then to high again through top to deep. The area of negative pressure water head decreased gradually due to rainfall and the absolute value of negative pressure water head reduces at the same time. The pressure water head keeps on increasing for 1 to 2 days owing to groundwater infiltration when the end of rainfall in some areas of slope. According to calculation results

maximal pressure water head in some areas of slope appears after two days when rainfall finished. Simulation results show that the model and program are reasonable; and it is of great importance in application to analyzing stability

designing drainage and reinforcing slope engineering.