Abstract: This paper analyzed the sources of water for desert riparian forests and identified the sources of water for different plant species. In order to explore the relationships among the plant species, the spatial differences in the sources of water for different forest plants were analyzed. The study provided the basis for locating ecological desert river forest oases with mixed plant species and for protecting inter-species interactions by monitoring aggressive water use and competition. A multi-isotope mass balance analysis was used to explore the potential contributions of different sources of water to plant. The xylem water and potential water resources including soil water and groundwater, of Populus euphratica (seedlings), Tamarix karelinii, Sophora alopecuroides, Hippophae rhamnoides subsp. sinensis, Karelinia caspia and Peganum nigellastrum were analyzed by using stable oxygen isotope composition (δD, δ¹⁸O) measurements and analyses. The results showed different δD, δ¹⁸O values in xylem water of different types of plants. This suggested that different plants absorbed water from different depths. The analysis showed that the main soil depth of absorbed water was 0-20 cm for P. euphratica seedlings, 200-300 cm for T. karelinii, 0-20 cm for H. rhamnoides subsp. sinensis, 50-100 cm for K. caspia, 0-20 cm for P. nigellastrum and 0-5 cm for S. alopecuroides. The contribution rates of different potential water resources were different to different plant species. Herbaceous plants mainly used shallow soil water. While shrubs with different plant species had different sources of water, P. euphratica also mainly used the shallow soil water. Based on the results of the study, competitions were noted among P. euphratica seedlings, H. rhamnoides subsp. sinensis and P. nigellastrum in the Heihe River desert riparian forests in terms of water use. P. euphratica and T. karelinii enhanced the water use of each other in the study area. Evaporation of soil moisture was the cause of enriched stable isotopes of hydrogen and oxygen in the lower reaches of Heihe River. However, these enriched stable isotopes were diluted by groundwater with increasing soil depth. This caused corresponding changes in stable isotope values with increasing soil depth.