Abstract: Water use efficiency (WUE) is an important index to evaluate plant drought resistance. Studying the dynamics of WUE and its influencing factors can provide reference for the vegetation restoration in Beijing mountainous area. We measured WUE of Platycladus orientalis in growing season and investigated the influence of meteorological factors, soil factors, and atmospheric CO₂ concentration on WUE, based on the stable carbon isotope techniques. The results showed that: 1) The short-term WUE decreased and then increased in the growing season, with minimum value (2.69 mmol·mol^-1) in July and maximum value (13.88 mmol·mol^-1) in October. 2) The vapor pressure deficit (VPD) had the most significant impacts on WUE, followed by air temperature (T_a), soil moisture (M_s), relative humidity (RH), and atmospheric CO₂ concentration (C_a), explaining 89.7% of the total variance. Solar radiation (R_a) and wind speed (W_s) had no impacts on WUE. 3) VPD and T_a are the most two important factors influencing short-term WUE, explaining 53.9% of the total variance. The effects of VPD on short-term WUE was higher than that of T_a. M_s and RH were the second important factors of the short-term WUE, explaining 25.4% of the total variance. The effects of M_s on short-term WUE was higher than that of RH. C_a had little effect on the short-term WUE and could explain 10.3% of the total variance.