Abstract: Based on the experimental data of evapotranspiration in the field, considering the meteorological conditions, cotton biological properties and soil moisture state, a calculation model for actual evapotranspiration of cotton field is developed by means of the Priestley-Taylor model, cotton leaf area index and relative effective soil moisture. The routine meteorological and agrometeorological data would be only needed to calculate the evapotranspiation by using the model, and there is a good precision. It can be used in arid area in China easily.