T. ramosissima nebkhas is an important component of wind-proof and sand-fixing forest in the lower reaches of Shiyang River, which plays a key role in protecting the oasis from sand disaster. Due to the deterioration of regional hydrological environment, a large area of T. ramosissima nebkhas has been degraded and died, which is in urgent need of artificial restoration, but its seasonal water source is not clear yet. In this paper, IsoSource model was used to quantitatively analyze water use strategies of Tamarix ramosissima shrub with different decline degrees based on hydrogen and oxygen stable isotope tracer technique. The results showed that the precipitation line equation in the lower reaches of Shiyang River in 2016 was δD=6.46δ¹⁸O-5.11,R²=0.87. Rainfall is mainly small rainfall events less than 5 mm, and the seasonal effect of rainfall δ¹⁸O is obvious. Soil moisture of T. ramosissima nebkhas with different degrees of decline was lower in summer and slightly recovered in autumn. The more severe the recession, the lower the water consumption. In the growing season, the order of soil moisture in T. ramosissima nebkhas was severe (3.48%) > extremely severe (2.69%) > moderate (1.97%) > mild (1.87%). The relationship between rainfall and soil moisture δD showed that rainfall from June to October contributed significantly to soil water supply in T. ramosissima nebkhas. Soil water, precipitation, groundwater were the potential sources of T. ramosissima nebkhas with different degrees of degradation. Due to the low evaporation loss, less canopy interception and soil biological crusts, the average proportion of precipitation utilization by T. ramosissima nebkhas in spring was 40.63%, and the order was severe recession (58.5%) > moderate recession (41.7%) > mild recession (39.3%) > very severe recession (23%). The decline of T. ramosissima nebkhas from spring to autumn tended to utilize reliable deep groundwater. Along with the water availability of habitat, T. ramosissima nebkhas in different degrees of decline selectively used rainfall, soil water and groundwater, which reflected its water adaptation and regulation strategy to cope with drought stress. Although it can barely survive, the declining T. ramosissima nebkhas in the lower reaches of Shiyang River had a tendency to be replaced by super drought-tolerant shrubs such as Reaumuria soongarica. It is suggested that in the construction of artificial vegetation in arid areas, water should be used to determine green, shrub and grass should be combined, and high water-consuming species such as trees and shrubs should be less selected as afforestation tree species.