Abstract: Three crop evapotranspiration affecting factors of soil,crop and atmosphere were calculated synthetically, combining with the actual situation of drip-irrigated cotton field with plastic film mulching in Xinjiang Region, the experiment with different film mulching degrees and cotton varieties was designed, the reference evapotranspiration was estimated by the model of Penman-Montieth, then the crop coefficients of different film mulching degrees and varieties were determined. Moreover, the goal of caculating evapotranspiration of drip irrigation with plastic film mulching in cotton field precisely based on it was reached. The results of experiment show that the whole growing period's evapotranspirations of drip-irrigated cotton field with plastic film mulching range from 540 mm to 620 mm, the evapotranspiration and crop coefficient are reduced with the increasing of film mulching degree, there are close correlations among leaf area index, daily evapotranspiration and crop coefficient,the changes of variety characteristics between different varieties lead to the distinctness of different crop coefficients.