As a key metric for measuring the dry matter yield gained per unit water used by plants, water use efficiency (WUE) has already become a hot topic for agro-forestry production and ecological research in arid and semi-arid areas. The WUE at the ecosystem level is usually defined as the ratio of the gross primary productivity (GPP) to the total evapotranspiration (ET). The ecology of the Qinling Mountains responds quite sensitively to climate change as a typical geographical transition zone and fragile ecological area. It has gradually become an important region in research on global change. To explore the variation in ecosystem water use efficiency in the Qinling Mountains and the responses of its ecology to current and projected climate change, we used five models (CCSM4, GISS-E2-R, GISS-E2-H, IPSL-CM5R-LR and NorESM1-ME) from the PCMDI database to forecast the trend of dynamic change in mean annual precipitation, mean annual air temperature, and ecosystem water use efficiency in the Qinling Mountains. During the period 2006-2100, four typical scenarios were studied (RCP2.6, RCP4.5, RCP6.0, and RCP8.5). The correlation between WUE and the key factors of climate change, including mean annual precipitation, mean annual air temperature, and CO₂ concentration, were analyzed. The results indicated the following: 1) The ecosystem-level WUE in the Qinling Mountains during 2006-2100 showed an obvious increasing trend in all scenarios, and the average tendency rate ranged from 0.0136 to 0.13 g C/kg H₂O every 10 years. With the growth in radiative forcing, the tendency rate and increasing range of WUE also increased. 2) The ecosystem-level GPP in the Qinling Mountains during 2006-2100 showed an obvious increasing trend in all scenarios; the average tendency rate ranged from 1.970 to 10.434 g C/m² every year, but the average tendency rate of the ecosystem-level GPP ranged from 0.338 to 0.738 kg H₂O/m² during 2006-2100 in these scenarios. Compared to ET, ecosystem-level GPP in the Qinling Mountains had a higher tendency rate, in these scenarios. 3) The tendency rate of mean annual air temperature ranged from 0.21 to 0.498℃ every 10 years at a highly significant level. The tendency rate of mean annual precipitation was about 7.78-17.66 mm every 10 years, but most results from the models showed insignificant upward trends. 4) Driven by these meteorological factors (e.g., air temperature, precipitation, and CO₂ concentration), both GPP and ET showed a rising trend; nevertheless, the increasing rate of GPP was more significant in comparison. Meanwhile, the subtropical evergreen broad-leaved forest on the south slopes of the Qinling Mountains has the tendency to move northwards, and may replace the warm-temperate deciduous broad-leaved forest now on the north slopes of Qinling Mountains, in response to climate change. Because of the increase in air temperate, precipitation, and CO₂ concentration, in addition to the increased leaf area index (LAI) of the ecosystem and the underlying succession of vegetation from 2006 to 2100, the ecosystem-level WUE in the Qinling Mountains showed an apparent trend of increase. The remarkable increases in air temperature and CO₂ concentration were the main driving factors for enhancing the WUE of the ecosystem in the Qinling Mountains, while precipitation was relatively insignificant.