Abstract: According to the measurement of soil moisture underground from zero to six metres of 12-year-old Chinese poplar, 13-year-old Chinese parasol, grassland and the corn land in Xinzhuang Village of Sanyuan, we studied the changes of soil moisture in different soil layers of 0-600 cm under different vegetation. The results show that from the upper to the lower, the soil moisture in artificial forest land changes from high to low, then to high in 2005 and 2006 with normal annual precipitation. The soil moisture in grassland and corn land is about 7% more than that in artificial forest land, and it become more and more downwards. After abundant annual precipitationl of 880 mm in 2003, the average soil moisture in the soil layer of 200-400 cm under 12-year-old Chinese poplar, 13-year-old Chinese parasols in 2005 and 12-year-old Chinese poplar, 13-year-old Chinese parasols in 2006 were higher than those in the dried soil in 2001, which indicates that if the precipitation were increasing, the dried earth layer would become non-dried earth layer. If the annual precipitation were up to 880 mm, the soil moisture could meet the normal growth of artificial forests for about 4 years. Guanzhong Plain and areas with similar annual precipitation are suitable for developing of forests basically. But because of the influence of weak dried earth layer, the forests can’t grow well. According to consuming amount of the soil moisture at 2% per year after the rainy year, weak dried earth layer will appear again under artificial forests in 2007 in Guanzhong Plain if mean annual precipitation is normal. The rainy year is the important supplying period of water for underground water in this area. Water seeping from soil to groundwater is normal in rainy year with a mean annual precipitation about 880 mm and lasting three years after the rainy year.