Abstract: According to the change rule of the soil moisture content and electrical RC (Resistance capacitance) parallel circuit,the change trend of soil moisture dynamic was analyzed and the formula of soil moisture was obtained by using the instantaneous unit hydrograph method. Through the comparison between soil gravimetric water content and modeled water content in Xiangxiang experiment station,the relative error less than 0.02 was obtained. It is proposed that the instantaneous unit hydrograph method for the estimation of soil moisture content is simple and practical,which can provide the theoretical basis for analysis of drought and soil moisture research.