Abstract: As the dominant coniferous tree species in the mountainous area of Beijing, it is imperative to have an insight into the evapotranspiration of Platycladus orientalis in the rapid growth season and its components for deeper understanding of atmospheric vapor exchange of terrestrial ecosystems and plant water demand. The variations of daily δ¹⁸O in a Platycladus orientalis plantation during the rapid growth season and proportions of plant transpiration and soil evaporation to evapotranspiration were analyzed. The continuous water vapor stable isotope analyzer was used to measure δ¹⁸O values of the atmospheric vapor in August, 2016. Mature and suberized twigs of plant and soil samples were collected and measured simultaneously to analyze δ18O values of transpiration (δ_T) and soil evaporation (δ_E), where δT and δE were determined respectively via the steady state hypothesis of isotope and the Craig-Gordon equation. The oxygen isotopic compositions of total evapotranspiration (δ_ET) could be estimated by a Keeling plot linear regression function.The results showed that: values of δ_E ranged from -5.968% to -2.689%, which rised firstly and then declined at the daily scale; and δ18O values peaked between 12:00 and 14:00, whereas the atmospheric relative humidity (h) dropped firstly and then rised. The relationship between δ_E and h was δM_E= -0.03h²+4.85h-209.5 (R²=0.55, n=32), which indicated that the isotopic fractionation was intensified with h when its value was above 75%. Although the diurnal variations of δ_ET and δ_T values were complex and there was inconsistent variation tendency for the two in the four experimental days (ranged from -1.210% to -0.951% and -1.599% to -1.004%, respectively), both of them basically shared identical changing trend at the same observation days, suggesting that the evaluation of δ_T values was probably affected by non-steady state of stable isotopes in the leaf water. And the variations on δ_ET were dominantly driven by δ_T rather than δ_E. The results of evapotranspiration partition showed that the contribution of plant transpiration (F_T) ranged from 90.14% to 92.63% during the experimental days, indicating that the main component of evapotranspiration was plant transpiration during the rapid growth season. The results may have further implications for future forest construction and management in this area.