Abstract: Based on the energy theory of soil water, this paper deals with the effect of watertight-ness of sand layer in loess soils through the lab experiments on water infiltration into soil column as well as mathematical simulation. The results showed that sand layer set up in the loess soils is characterised not only by better water-tightness, which can make infiltration flow stay over sand layer in soil body to a certain limit exent so as to improve water retaining capacity in upper soil layer, but also by infiltration flow as well as the wetting velocity and also convert the non-linear process of infiltration into the linear process, thus resulting in the whole infiltration process to enter the stable infiltration stage with a smaller infiltration rate. Accordingly the results of this project have provided the theroretical base for the adoptation of setting up sand layer in the loess soil in Northwest China as an important technical engineering practice for the inflitration prevention and reduction to water conservancy work and architectural engineering as well as cave-dwellings of the local inhabitants.