Water shortage is the main limiting factor for vegetation restoration and desertification control in the arid region of northwestern China. 【Objective】Investigation of the root water source and influencing factors of typical plants used for desertification control can provide a basis for scientific sand control and efficient water use in the region.【Method】Pinus sylvestris L. var. mongholica Litv. (P. sylvestris) is an evergreen coniferous tree of the Pinus genus. It is widely used in wind sheltering and sand fixation in the north of China because of its strong root system and ecological adaptability. In this study, the hydrogen and oxygen isotope composition (δ²H and δ¹⁸O) of plant xylem water and its potential water sources (precipitation, soil water and groundwater) were measured monthly from June to November 2018 in an artificial P. sylvestris forest (18～20 a). The experiment was carried out in the Gechougou watershed in the southeastern part of Mu Us Sandy Land. The dynamic changes and controls of root water sources were studied by the multiple linear mixed models. 【Result】Our results showed that the range of δ²H was –69.95‰～ –49.25 ‰, –144.81‰～ –6.60‰, –83.62‰～ –48.57 ‰ and –65.63‰～ –53.65 ‰ for xylem water, precipitation, soil water and groundwater, respectively. Also, the range of δ¹⁸O was –8.77‰～–8.21‰, –18.86‰～–2.07‰, –9.45‰～–6.54‰ and –9.97‰～–8.26‰, respectively, during the period monitored (June–November). The weighted mean values of δ²H and δ¹⁸O in soil water were between the precipitation and groundwater due to the influence of precipitation infiltration and groundwater recharge. Rainfall amount, soil water content and groundwater level in the rainy season (July–September) were 24.80～90.10 mm, 3.36%～8.40% and 5～15 cm higher than that in the dry season (June, October, November). Iso-source model predicted that P. sylvestris mainly used the nearly saturated layer water (>90 cm) (15.40%) and groundwater (70.10%) in June. However, from July to September, the root used water source mainly came from the active and stable soil layer (<80 cm) (61.03%) with the increase in rainfall. Compared with the rainy season (July–September), the contribution of the nearly saturated layer (>70 cm) and groundwater to the root water source increased by 5.82%～28.00% and 20.64%～23.30% in October and November when precipitation decreased. During the detection periods, the average ratio of absorption and utilization by P. sylvestris root from the active layer, stable layer, nearly saturated layer and groundwater were 21.48%, 31.13%, 16.42% and 30.98%, respectively. The dependence on groundwater for P. sylvestris was significantly higher than that of other soil layers, which may be prejudice to survival under extreme drought environments in the future.【Conclusion】 The soil water content was positively correlated with the root water absorption of P. sylvestris. The uplift and decline of groundwater level significantly changed the position of the nearly saturated soil layer, and then affected the absorption and utilization ratio of P. sylvestris from different potential water sources. Therefore, the root water source of artificial P. sylvestris forest was affected by both supply-consumption dynamics of soil water and seasonal fluctuation of groundwater level. This study provides a useful method that identified the absorption and utilization ratio of water by plants from different potential water resources, which will be significant for vegetation restoration in the Mu Us Sandy Land soil-plant ecosystems.