Abstract: Improvements in water use efficient and limited groundwater consumption are important for water sustainable resource development. Atmospheric water vapor exchange sufficiently indicates water consumption in irrigated agro-ecosystems. This paper clarified the changes in water vapor isotopic composition with height and time of the day or longer periods. The relationship between the quantity and isotopic composition of evapotranspiration vapor was also determined. The chamber method (which was based on high frequency and real-time water vapor stable isotope analyzer) was used to measure isotopic composition of water vapor at different heights and times of the day/period. Using water vapor flux of crop land measured with Eddy Covariance method, the relationship between isotopic composition and evapotranspiration was also determined. Based on the different time scales, the results showed that water vapor isotopic composition at the ecosystem boundary layers continuously fluctuated during several days. The daily changes were stable with smaller fluctuations at hourly time-scale. There was a steady enrichment in daily changes in isotopic composition under atmospheric background. No obvious trend of change was noted in isotopic composition of the day. With no direct and rapid canopy effect, isotopic composition under atmospheric background was stable at hourly time-scale. There was also an obvious enrichment of chamber isotopic composition of evapotranspiration. Isotopic compositions of evaporation and transpiration were similar for midnight and clearly dissimilar during early morning and noon. Due to isotopic fractionation, isotopic composition of evapotranspiration water was heavier than the vapor at 2 m height. A good linear correlation was noted between the quantity and isotopic composition of evapotranspiration with correlation coefficient of 0.87 and regression equation of y = 53.21x-115.06. The study typified the vitality of evapotranspiration research via the isotope method.