Abstract: Research on the water sources of different ages of Populus euphratica plays a key role in formulating the conservation strategy of Populus euphratica. This study discusses the contributions of different potential water sou- rces to Populus euphratica，based on the composition of the stable oxygen and hydrogen isotope (δ¹⁸O，δD) of the xylem in different ages of Populus euphratica and different potential water sources in the lower reaches of the Hei River. The results showed as follows：（1）there were significant differences of δ¹⁸O value in the xylem of different ages of Populus euphratica. The δ¹⁸O values of the seedlings，mature wood and over-mature wood of Populus euphratica reached -5.368‰，-6.033‰ and -6.924‰，respectively.（2）The water utilization of different ages of Populus euphratica came from different sources. The seedlings of [Populus euphratica] mainly used the soil water in 30-50 cm layers，with the utilization rate of 70%，while the utilization rate of groundwater only about 6%. The mature wood of Populus euphratica mainly used the soil water in 200-320 cm layers and the groundwater，with the largest utilization rate at 85% of the groundwater. The over-mature wood of [Populus euphratica] mainly used the soil water in 100-260 cm layers and also the groundwater，with the higher utilization rate (about 96%) of the groundwater. And the depth range of the over-mature wood is wider than that of seedlings and mature wood of Populus euphratica.

Key words: water sources, different ages of Populus euphratica, D and ¹⁸O stable isotopes, the Hei River