Un-deterministic dynamic of the energy process, water recycling and biological process in farmland is leading from its stochastic components, such as precipitation, evapotranspiration, heterogeneity of soil, topography and so on. The soil water balance is often described as a reservoir that is deterministically depleted via water losses due to evapotranspiration, runoff, leakage, and stochastically receives water from precipitation, so the stochastic property is the inherent nature of soil moisture. Based on the data of soil moisture, precipitation, meteorology and crop from 2000 to 2008 in Luancheng experimental station of agro-ecosystem of CAS on the stage of summer maize growth, the simulation was carried out with the stochastic model for soil moisture dynamics of the farmland at a point in piedmont plain of Taihang Mountain by using Laio model. The results show that the frequency and mean amount of daily rainfall from 2000 to 2008 in study area on the stage of summer maize growth are 0.2909, 10.71mm, respectively. And different changing trends were found among the frequency occurrence of different rainfall levels, the light rain and storm showed sharply increase trends, and the opposite trends was found in the moderate rain, for the heavy rain, the trends was slightly decrease. The revolution of soil water content of the growth stage of summer maize was expressed as increasing in June, stable in July, and decreased from August to September, and the maximum of soil water content was found in July, the value was 32.2%. A good agreement between the result of modeling and observation was found based on the curve shape of probability density function which included the curve peak value, the position of the peak, the confidence interval of 90% and the digital characteristic of the relative soil moisture which included the median value, expected value, and the variance(α=0.05). And the Laio model had a good application in the irrigated farmland, and the irrigation was treated as a rain. The amount of the irrigation which could made the soil moisture remained above 80% of field capacity on the stage of the summer maize growth under any loss in irrigation activity conditions was calculated by the Laio model, and the value was 32.1mm at the probability of 50%.