Using four groups of survey data of soil moisture of winter wheat from returning green stage to harvest stage in Daxing region, the variation laws of soil moisture exponential depletion relations in 0-60cm depths during different growth stage are analyzed ,and based on soil moisture exponential depletion relations the forecast model of soil moisture depletion is built. The results indicate that the soil moisture exponential depletion relations in the same depth layer increase at first and then decrease. Moreover, for different depth layers they decrease with the increase of depth, and the dispersion degree also changes with the depth. Using the exponential depletion relations method, the soil water storage at the 20, 40 and 60cm depth layers are forecasted, and the minimum forecast error is 0.09%, the forecast accuracy in the layer of 60cm is highest, and the absolute error is between 0-1.5%. This method does not need much data, and its results are reliable and precise. So it is a practical method for soil moisture forecast in winter wheat field of Beijing and others whose soil condition is same as Beijing.