Abstract: The dynamic laws of soll moisture in hilly areas of the Loess Plateau are studied in this paper．The results show that the precipitation is the most important factor to determine the tempo ral change of soil moisture；the soil moisture decreases from Ocm to 30cm and increases from 30cm to 120cm and the total changing trend is slowly；the ranges of spatial variability of soil moisture for bareness and farmland are 3．17m and 7．25m respectively．