Based on field investigation in the Guanzhong plain and Baiji area, this study analyzed available water content, distribution of gravity water，water cycle and balance in the first paleosol layer (S1) using laboratory analysis. The results show that the migration depth of CaCO3 and Sr is up to 42m. The development thicknesses of weathering loess under the bottom of argillic horizon and leaching fracture are 22 m and 39 m respectively. The migration depth of red ferruginous clay adhesive film rich in Fe2O3 is 1m under the bottom of argillic horizon. The crystal calcite of CaCO3 concretion is well developed. These indexes indicate that the developmental condition of the first paleosol layer was warm and humid, the distribution depth of gravity water is up to 42m at least and the content of water is adequate in the range of 42m. The average moisture content of S1 is about 22% during vegetation growing season and in most time every year. Meanwhile, the available water content is about 14% after evaporation, absorption, crown interception and surface runoff loess. When there was sufficient soil moisture and no dried earth layer developed in the Guanzhong Plain, paleosol can provide enough water to satisfy the need of forest to grow. The distribution depth of gravity water during the development of S1 was 2 m thicker than that nowadays and the soil water was acid. The input volume of soil water was higher than output volume during the development of S1, indicating the value of soil moisture balance is positive and atmospheric precipitation during rainy season could supply the underground with much water. This was why there is high content of soil water at that time. The fact that the distribution depth and content of gravity water in Guanzhong Plain and Luochuan ws sufficient in 2003 can demonstrate that the research results of soil water during the development of S1 is reliable. These indexes indicate that, during the most humid and moisture stage of S1 paleosol development, summer atmosphere surpassed the Qingling Mountains to the Guanzhong plain and the subtropical climate was prevailing over the northern and southern sides of Qingling Mountains, suggesting the Qingling Mountains be no longer the boundary between the subtropical zone and the temperate zone in China. The development of the first paleosol layer lasted more than 50,000 years and it is possible to develop forest steppe vegetation outside the range of humid and warm. This study not only broadens a new content for Quaternary research but determines the important application value of paleowater content in paleosol study at home and abroad.