Groundwater-dependent terrestrial vegetation (GDTV) plays a crucial role in maintaining oasis ecosystem stability in arid regions. This study investigates the water-use strategies of three typical GDTV species in the lower Tarim River, China. We analyzed the thydrogen (delta D) and oxygen (delta 18O) isotopic compositions of plant xylem water and potential water sources, using the MixSIAR model to quantify source contributions during the growing season. Results indicate that soil water isotopes exhibited significant vertical gradients and seasonal dynamics, with enriched values in shallow layers and variable, depleted values in deeper layers. In contrast, groundwater remained isotopically stable. The three species displayed distinct water-use strategies: Populus euphratica primarily relied on groundwater (contributing up to 73% in summer) and deep soil water; Tamarix ramosissima mainly utilized deep soil water (peaking at 68%); whereas Launaea polydichotoma depended on shallow to middle-layer soil water with marked seasonal shifts. Furthermore, soil moisture and salinity gradients were identified as key drivers of root differentiation, shaping the spatial and temporal patterns of plant water use.These findings provide a scientific basis for understanding the eco-hydrological mechanisms of GDTV in arid ecosystems.