Based on the theory of gas exchange measurement of chamber system, the paper used Li-8100A automated soil CO2 flux system to measure the diurnal evapotranspiration of alpine meadow community in the upper reach of Heihe River. The relationship between the diurnalvariation of evapotranspiration rate and environmental factors was represented by the correlation coefficient and path coefficients. The result shows that the Li-8100A can be effectively used to measure the changes of diurnal evapotranspiration rate.The variation of alpine meadow evapotranspiration rate appears single-peak changing tendency over time in a sunny day.There are larger difference between the sample with grass land and that with bare land;the difference of the two could be considered as the grass transpiration. The correlation coefficient and path coefficients between the evapotranspiration rate and environmental factors indicated that the air temperature has the strongest positive effect on evapotranspiration rate, the subsequent factors is soil moisture and wind speed. In addition, the soil temperature, solar radiation have indirect effect on evapotranspiration rate through the change of air temperature.