Knowledge on the transpiration and environmental factors of forests facilitates the management of self-sustainable vegetation ecosystems in northern rocky mountainous areas, and provides scientific vegetation construction strategies. We surveyed forest transpiration and environmental factors in a mixed stand of Robinia pseudoacacia and Pinus tabulaeformis from July 11 to October 31, 2016 in the experimental forest of the Beijing Forestry University, West Mountain. A thermal dissipation probe(TDP) system was applied to monitor the transpiration. To study the effects of the environmental factors on transpiration, volumetric soil water content, and meteorological variables were also monitored simultaneously, including solar radiation(R_n), air temperature(T), relative humidity(RH), wind speed(W), and precipitation(P). This study aimed to(1) analyze forest transpiration processes and the transpiration responses to environmental factors, and(2) determine the canopy coupling of the forest and quantify the contribution of different factors to transpiration. Results showed that(1) the two species exhibited different seasonal transpiration trends despite their similar diurnal patterns. The transpiration of R. pseudoacacia decreased in autumn. By contrast, the transpiration of P. tabulaeformis was maintained throughout the growing season. The two species demonstrated similar transpiration patterns, but the transpiration of P. tabulaeformis was higher than that of R. pseudoacacia;(2) Both species demonstrated hysteresis between transpiration and R_n. But R. pseudoacacia had a clockwise hysteresis loop, whereas P. tabulaeformis had an anti-clockwise loop. Hysteresis was also observed between transpiration and VPD(vapor pressure deficit). Both species showed clock-wise hysteresis loops with much higher transpiration rates in the morning than afternoon;(3) The main environmental factors influencing transpiration included R_n(P < 0.01), VPD(P < 0.01), and W(P < 0.01). Among them, VPD induced higher transpiration than R_n;(4) the two species were well coupled to the ambient environment(Ω < 0.1); thus, their stomatal control over transpiration was efficient, and the effect of R_n was highest before noon. The sap flow was observed to start earlier under high VPD conditions to avoid xylem cavitation or embolism owing to intense transpiration;(5) The soil water condition of shallow layers(0-50 cm) was not a major influencing factor on plant transpiration. Therefore, further study should include monitoring deeper soil layers to locate the soil water supply source. Based on our results, the transpiration of this mixed stand could be reduced through the abatement of canopy conductance. Thus, water restrictions would be alleviated and the survival rate would be increased. Both species could efficiently physiologically control transpiration. Therefore, in practice, the water consumption of this forest could be managed by reducing the canopy conductance. This could be carried out by pruning, or selecting a species with a low canopy conductance to increase the survival rate of the forest under limited water conditions.