Starting from the soil water balance equation for rainfed fields,a dynamic model is constructed for arid-land soil water regime with the chief budget terms considered in a comprehensive fashion and necessary improvement done on the percolation estimate. For the water change on a decade basis,simulation has been undertaken in terms of the data from non-irrigated land measured by Tianstui Weather Station,Ganshu and Jingyang Station,Shanxi,both located in semi-arid climate. Restilts show that the modelling has yielded good outcome that best describes the soil water change in these regions.