Water is one of the essential substances to maintain the plant system. Water resource selection is an important selection process in the botany cycle. Despite precipitation is the ultimate water source for all plants, factors like evaporation,difference in altitude,geological aquifer features, differences in the soil water characteristics may all influence both values of stable hydrogen and oxygen isotope (deltaD, delta~(18)O). Where the plant extracts water is supposed to be revealed by comparing the water delta~(18)O、deltaD values from stem xylems with that from the available water sources. Qaidam Basin is located in the northeast of Tibetan Plateau with its typical desert ecosystem being significantly sensitive to global change. Nevertheless, there is no information found on how its plant system distributes their water exploration among the available water sources in that area. How the river water and precipitation influence the plant water sources is unknown. The objective of this study is to determine the proportions of different water sources drawn by selected dominant plant species,examine the consistence of this proportion for different plant species during the growth season,investigate the water competition conditions among the selected dominant species. Five local dominant plant species (Ceratoides latens, Nitraria tangutorum, Ephedra sinica, Tamarix ramosissima, Calligonum mongolicum) were selected to test the stem water delta~(18)O and deltaD using Picarro L-2130i and Finnegan MAT-253. delta~(18)O and deltaD was also tested for different available water resources including groundwater, river water,soil water (profiled as 0-10 cm, 10-30 cm, 30-50 cm, 50-70 cm,70-90 cm) and effective precipitation at same time, as sampled every 15 days during June to September in 2010 in Nuomuhong Area. Meanwhile Decagon's Em50 Em50R was used to measure soil water content. The Multi-source mixed linear model (IsoSource) was used to analyze the contribution of different water sources to these desert plants. The local meteoric water line (LMWL) equation was y = 7.019x-3.217(R~2 = 0.970, P<0.001), which indicated the warm and dry climate features of the study area. The result of delta~(18)O and deltaD analysis showed that the selected plant species mainly used soil water in the whole growth season. The groundwater was the second major water sources. Ceratoides latens mostly used soil water in 10-50 cm. Nitraria tangutorum and Tamarix ramosissima absorbed most soil water from 50 to 70 cm. Soil water competition existed in 50-70 cm between these two species. Ephedra sinica and Calligonum mongolicum used soil water from each layer more evenly. So their water competition exists in the whole profile. Nitraria tangutorum, Ephedra sinica,Tamarix ramosissima and Calligonum mongolicum changed the absorbing proportion of different water sources during growth season. Groundwater utilization is increased gradually in the test season as the plant grows,which indicated that groundwater is an important water source to the desert vegetation. Thus maintaining the groundwater tables in arid and semi-arid area is important for desert plant system. For water scarce region, desert shrub root phenotype or spatial structure is an important aspect for root functioning. Mostly plant root system distributed throughout the whole soil profile. It does not always coincide with the water abstracting profile. So the way of judging the plant water sources only based on plant root profile has its limitation.