Abstract:The hydrogen and oxygen isotopic compositions (δD and δ18O) of stem water and its potential sources (rain water, soil water, and groundwater) were measured monthly from June to November, 2018 in a Salix psammophila forest (18~20 years) in Gechougou watershed of Mu Us Sandy Land. The dynamic changes and controls of root water source were studied by the multiple linear mixed model (IsoSource). Results showed that the variation range of δD were -82.41‰~-52.91‰, -144.81‰~-6.60‰, -96.94‰~-42.65‰, and -86.42 ‰~-71.82‰ for stem water, rain water, soil water, and groundwater, respectively. The variation range of δ18O were -10.77‰~-7.29‰, -18.86‰~-2.07‰, -12.85‰~-0.79‰, and -10.86‰~-9.74‰, respectively, during the monitored period (June-November). Rainfall amount and soil water content in the rainy season (July-September) were 24.80~90.10 mm and 0.95%~1.84% higher than that in the dry season (June, October, and November). However, the groundwater level during June to September were 2~7 cm lower than that in October and November. In June, S. psammophila mainly used deep soil water (>200 cm) (33.70%) and groundwater (26.20%). From July to September, the root water source mainly came from shallow soil water (<200 cm) (50.70%~54.00%). Compared with the rainy season (July-September), the contribution of shallow soil water (<200 cm) to root water source remarkably increased by 35.20%~40.00% in October and November due to the low air temperature, rainfall amount, and weak life activity of S. psammophila. There was a significant reduction in the use of groundwater in October and November (<5.00%). Therefore, S. psammophilais adaptable to the seasonal drought and is suitable for desertification control in the Mu Us Sandy Land. The root water source of S. psammophila changed with seasonal variations of rainfall amount, soil profile water content, and groundwater level.