The quaternary sediments overlying on the sandstones in the Yungang Grottoes with a certain of thickness are able to slow down surface water infiltration and reduce the damage of the grottoes from water intrusion. The gravel sand layer in the lower part of the sediments is not important because of its high permeability, whereas the upper silty soil layer plays an essential role for stopping infiltration due to its low permeability. The thickness of the silty soil is the key factor. When a surface pond of water exists, based on artificial infiltration test, it is indicated that the water would quickly move to the top of sandstone if the thickness of the silty soil is less than 0.7 m, however it could be effectively blocked if the thickness of the silty soil is greater than 1.1 m. Surface water could also quickly infiltrate into a thin silty soil and then have influence on the grottoes through lateral seepage. The root canals of plants would significantly speed up the infiltration rate. Accordingly, in order to reduce the risk of water intrusion, planting trees on the top of Yungang Grottoes should not be encouraged.