Evapotranspiration is mainly affected by energy,soil moisture content,and biological characteristics of plant.Based on the field experimental data for years,an empirical model to estimate the evapotranspiration of Populus tomentosa shelter belt from April to October in cropland of the Alaer irrigation area is established by the way of mathematical statistics method.Results show that the ratio of the evapotranspiration to water surface evaporation and the soil moisture content in root zone display the Logistic curve.The model uses routine meteorological and soil moisture data only,so it is convenient in calculation and valuable in utilization.