###### Abstract  
Water use efficiency (WUE) reflects the balance between carbon assimilation and water consumption in terrestrial ecosystems. Considering the fragile conditions of global water resources, the evaluation of regional WUE variation characteristics and response mechanisms is critical for promoting sustainable ecological development and water resource utilization. Based on gross primary productivity (GPP) and evapotranspiration (ET) datasets, combined with vegetation and meteorological data, this study examined the spatiotemporal variations, annual variation contribution rate, and driving mechanism of WUE in Inner Mongolia from 2001 to 2020. The main results are as follows: (1) The interannual and spatial variations of GPP, ET, and WUE all exhibited increasing trends, with WUE increasing in approximately 70% of the region and significantly increasing in 22.35% of pixels exhibiting a significant increase. Areas with the most significant increases were located in the Horgin sandy land and Mu Us sandy land. (2) The highest WUE values were observed in the summer season, followed by autumn and winter, and the lowest in spring. (3) Among all vegetation types, the typical steppe ecosystems contributed most to the interannual variability (IAV) of GPP, ET, and WUE, with values of 169.89%, 141.09%, and 193.42%, respectively. While the coniferous forest contributed least or negatively to GPP, ET, and WUE IAV, with values of \(-\)36.28%, 28.20%, and \(-\)32.86%, respectively. (4) The primary driver of WUE variation was found to be GPP, which contributed 59.36%, mainly in the central and western regions. The remaining 40.64% was attributable to ET, concentrated in the northeast region. (5) Human activities significantly affected WUE, with a contribution (about 53.52%) larger than that of climate change (nearly 46.48%). Increased precipitation improves vegetation WUE and is the most important climate factor influencing WUE variations. These findings will aid the formulation of vegetation protection and water resource management strategies in water-stressed areas. water use efficiency; interannual variability (IAV); relative contribution; sensitivity analysis 15 September 2022 25 October  
via photosynthesis and represents the largest carbon flux in the global terrestrial carbon cycle [9; 10; 11]. ET is an important water vapor flux that refers to the water used by an ecosystem through transpiration or surface evaporation and influences water and energy distribution between the land surface and the atmosphere [12]. The study of vegetation WUE can not only provide an in-depth understanding of ecosystem change patterns but also explain how ecosystems respond to climate changes and water resources, laying the groundwork for promoting vegetation productivity and ecological construction in water-stressed areas. Numerous studies have shown that the variability of WUE is related to a variety of factors, including human activities (vegetation greenness) [13; 14; 15], climate factors [16; 17], CO\({}_{2}\) fertilization and N deposition [18], canopy conductance [19], and drought [6]. In arid ecosystems, WUE variability is mostly influenced by human activity and climate factors [20; 21; 22]. For example, [PERSON] et al. [21] found that WUE variations in the arid ecosystems of China were controlled by precipitation. [PERSON] [22] suggested that precipitation promoted vegetation WUE in dryland regions of China. [PERSON] et al. [20] pointed out that WUE in northern China presented a dominant increasing trend and WUE variability was primarily controlled by climate factors (precipitation, temperature, and solar radiation), with climate accounting for 84% and human activities accounting for approximately 16%. Nevertheless, [PERSON] et al. [5] found that human activities played a critical role in the increase of WUE in the Beijing-Tianjin Sandstorm Source region, with a relative contribution of 88.2%, followed by precipitation and temperature, which contributed only 11.8%.