Abstract: On the basis of the experiment al data in farm land,a function relationship between the potential evapot ranspiration and the relative water content of the soil in the root zone wasest ablished.By using the function relationship,the actual evapot ranspiration was cal culated in natural rainfal condition in Huang-Huai-Hairegion.Theannual change of the actual evapot ranspiration was in dual-peak type,the first peakvalue occurred in the booting-flower stage of wheat,the second one took place during booting-flower stage of summer corn.The trend of regional distribution of evapotranspiration conformed to that of natural precipitation,its value was over 84% of the rainfall.