In this paper, stochastic approach is applied to research soil moisrure quantitatively, and it is supposed that the soil water series can be represented by a general additive model and it proved that soil moisture series is supper posed by a periodic-deterministic process and a stochastic component. The periodic component is the effect of climates periodic fluctuation and crop evapoti anspiration, and the stochastic component is the effect of stochastic fluctuction of climate on soil moisture. Results of modeling the soil water process ar Wugnng. Shanxi, Yongji Shanxi, Fufeng. Shanxi has demonstrated that this method for modeling soil water content series is of higher precision.