Abstract: Based on the soil moisture data from 1980 to 1992, according to the principle of soil water balance and the optimization technique in operational research, and considering the special natural conditions and actual state of crop growth in Jiangsu Province, the calculating methods of effective rainfall and the amount of complement water of low level soil are given, and the calculating models of water budget of dry farmland and rice field are proposed. By using these models and real-time rainfall and soil moisture data, the amount of farm water budget can be calculated accurately, so that the effective service to agrometeorological information would be provided.