Water plays an important role in plant growth and distribution, and its resource selection is an important process in the botanical cycle. Although precipitation is the ultimate water source for all plants, factors like evaporation, altitude, geological aquifer features, and soil water characteristics may all influence values of stable hydrogen and oxygen isotopes (δD, δ¹⁸O). The water source used by a plant can be revealed by comparing the water δD and δ¹⁸O values from stem xylem with those from the available water sources. Subalpine shrubland is an important vegetation type in the Qilian Mountains, and has high water-conservation capacity. Studies on the water source of the dominant plants of the subalpine shrubland in the Qilian Mountains contribute to understanding the water cycle and eco-hydrological process of subalpine shrubland. In this study, six species of dominant subalpine shrub plants in the eastern Qilian Mountains were selected to analyze the water sources of different plants in the same biotope landscape and the water source changes of the same plant in different growth stages using multivariate linear mixed models (Isosource). The local meteoric water line (LMWL) equation was δD=7.775 δ¹⁸O + 12.34 (R²=0.871, P < 0.001), which showed that the study area has the characteristics of strong evaporation, low temperature, and high humidity. The results showed that the six dominant plant species mainly used precipitation during the entire growth season. Soil water from 0 to 10 cm was the second major water source. The contribution of groundwater was the lowest. The six dominant plant species changed the proportions of different water sources utilized during the growth season. In July, the six dominant plant species mainly used precipitation and shallow soil water. The contribution rates of precipitation and 0-10cm soil water to Spiraea salicifolia L., Rhododendron capitatum Maxim, and Rhododendron thymifolium Maxim were similar, but the other three species mainly used precipitation. In August, the main water source of all six species was precipitation. The use of soil water in each soil layer from 10 to 80 cm by R. thymifolium and S. salicifolia decreased over time, whereas that by the other four plants first increased and then decreased with depth. The ability of S. salicifolia and R. capitatum to utilize soil moisture was strong. The use of groundwater by R. capitatum was higher than that by other plants.