Climate warming and human activities have altered water resource distribution in desert areas, accelerated water infiltration and evaporation, disrupted the water cycle, and increased drought stress in riparian forests. However, the effects of climate varying environmental conditions on intrinsic water use efficiency (iWUE) and tree growth in Populus euphratica Oliv. remain unexplored. Using tree-ring width and δ¹³C composition data, we analyzed the growth and iWUE of P. euphratica across three sample plots with varying groundwater depths in the Daliyaboyi Oasis. We found that P. euphratica basal area increment (BAI) and iWUE significantly increased across all sampling plots from 1980 to 2003 with no significant changes from 2004 to 2022. In the shallow-groundwater sample plot, P. euphratica BAI and iWUE were significantly positively correlated with temperature (TEM), vapor pressure difference (VPD), and runoff (RO) and significantly negatively correlated with relative humidity (RH). Conversely, in the deep-groundwater sample plot, P. euphratica iWUE was significantly and positively correlated with RH and negatively with VPD. The correlation between the intercellular and atmospheric CO₂ concentration ratio (C_i/C_a) and meteorological factors shifted from non-significant to significant with increasing groundwater depth. C_i/C_a variations in P. euphratica were significantly positively correlated with TEM, VPD, and RO and significantly negatively correlated with RH. Thus, with continued climate warming, P. euphratica in deep-groundwater sample plots may face severe drought stress and growth declines in the future. Understanding the dynamic relationship between iWUE and BAI under varying environmental conditions is crucial for predicting forest responses to climate change and informing conservation strategies.