In order to study the mechanism of agricultural soil moisture conservation through sand injection in the northwest arid region of the Loess Plateau, rainfall infiltration tests on three kinds of indoor soil columns(a homogeneous soil column, a soil column with a 3 cm-thick sand layer, and a soil column with a 5 cm-thick sand layer)were performed, and the water-resisting phenomenon of the sand layer was analyzed through establishment of a micro model. The water contents in different layers of soil columns at two times(13 days and 197 days)and the δD and δ¹⁸O values of soil water were measured with the drying method and MAT253 gas isotope ratio mass spectrometer. The change in water content with the depth and the relationship between δD and δ¹⁸O of soil water were obtained. The results show that the sand layer can retard water infiltration rather than prevent water infiltration. The reasons for this are that the water-holding capacity of sand is far less than that of loess, retarding water infiltration, and the sudden decrease in the wetted cross-sectional area at the interface with abruptly changed medium causes the water infiltration velocity to decrease.