Tamarix ramosissima and Haloxylon persicum in the desert-oasis transition zone of the southwest edge of the Gurbantunggut Desert were used as research objects, and the δ¹⁸O values of plant xylem water and potential water sources were analyzed in combination with the MixSIAR model, and the water sources and utilization ratio of each water source of T. ramosissima and H. persicum in different communities were analyzed. The results showed that: (1) The δ¹⁸O values of soils in the 0—60 cm layer under T. ramosissima and H. persicum thickets varied widely, and the δ¹⁸O values tended to be stable with the increasing of soil depth, indicating that the surface soil was greatly affected by the external environment. (2) In the symbiotic communities of T. ramosissima and H. persicum, T. ramosissima mainly used soil water in the 300—500 cm soil layer throughout the growing season, and H. persicum mainly used soil water below the 60 cm soil layer in spring and autumn, and used soil water in each layer more evenly. It mainly obtained soil water in the 0—60 cm soil layer in summer, and the utilization ratio was as high as 88.0%. It showed that there was ecohydrologic niche separation of water use strategies between T. ramosissima and H. persicum, which was conducive to their rational use of limited water resources under water deficit conditions. T. ramosissima had similar water use sources in single and symbiotic communities and mainly obtained stable deep soil water, indicating that the root water absorption of T. ramosissima was insensitive to fluctuation of the external environment and had no obvious seasonal variation in water consumption. (3) The single H. persicum community made more uniform use of soil water in each layer in spring, and mainly absorbed soil water in the 300—500 cm soil layer in summer, and the utilization ratio was as high as 76.4%, while in autumn, it mainly used soil water in the 60—300 cm soil layer. Artificial H. persicum communities mainly utilized soil water in the 0—60 cm soil layer in spring and summer with contribution rates of 64.2% and 80.6%, respectively, while they mainly absorbed soil water in the 120—300 cm soil layer in autumn with a utilization rate of 93.9%. The result showed that the natural growing H. persicum had a relatively wide range of water sources, reflecting its strong adaptability to arid environments, while the artificial cultivated H. persicum was sensitive to precipitation and showed opportunistic characteristics in water use.