Taking the Platycladus orientalis forest distributed widely in Beijing mountain area as research object, the evapotranspiration of the stand was quantitatively separated by two methods, which were hydrological measurement method (sap flow sluice combined with large-scale lysimeter) and stable isotope. The results showed that:(1) On the daily scale, the evapotranspiration and transpiration of the stand both showed "single peak" type curves. The total evapotranspiration and transpiration of the stand reached the maximum values around noon, which was 1.27 mm/h and 1.13 mm/h, respectively. (2) The proportion of transpiration to total evapotranspiration calculated by the measured method and the stable isotope method was 80.21%~89.63% and 79.10%~98.71%, respectively. Compared with the measured values, the error of the stable isotope method was(3.97±3.53)% on the hourly scale, and was (1.89±0.67)% on the daily scale. The evapotranspiration of this stand was mainly from the transpiration of vegetation, and the water consumption of forest transpiration was far greater than that of soil evaporation.