Because of the special capillary and water-sensitive characters, the moisture content of silt subgrade in Alluvial Plain of the Yellow River is generally higher than the optimum value, which will lead to bad support conditions of subgrade. In order to analyze the water-absorbent property of silt subgrade, both the rainfall seepage and the capillary phenomenon were simulated in laboratory, which reveals that both factors have distinct effects on the moisture content of compacted silt soil. Triaxial tests of compacted silt soil reveal that the cohesive force reduces obviously with the increasing of moisture content. What’s more, the internal friction angle also reduces rapidly arriving at saturation state. By adopting Geo-slope code, the rainfall infiltration mode of silt subgrade was simulated under different rain type and rain duration. Results show that the subgrade slope is easy to lose stability because of much rainfall infiltration. In addition, the minimum safety coefficient happens at the time of rainfall ending. And the slope safety can be threatened seriously by the repetitious rainfall.