A water balance sub model of crop growth model,CERES wheat model,which was established originally by American scientist J.T. Ritchie et al.,was applied to simulate soil moisture of a fixed winter wheat plot at the Xifeng Agrometeorological Experimental Station,Gansu Province,located in semi arid zone. Corrections were made to estimate potential evapotranspiration,soil surface evaporation,crop transpiration,and root density and maximum rooting depth. Both the density and depth were represented as two functions of crop developmental stages respectively. The improvement achieved good results in soil moisture prediction and may provide a convenient and practical method for management of croped soil moisture in arid or semi arid zone.