Using the leaves of Populus euphratica forest age groups (young trees and mature trees) from eight different habitats in Xinjiang as material, we measured natural stable carbon and nitrogen isotope composition (δ13C and δ15N) value, carbon content, nitrogen content of leaves and specific leaf area, analyzed its δ13C and δ15N values with altitude, latitude and longitude, leaf carbon and nitrogen contents, specific leaf area as well as the relationship between water use efficiency. The result showed that: (1) the average δ13C value of sapling and mature leaves was -27.863‰(-28.776‰ - -26.695‰) and -28.230‰(-29.717‰ - -26.033‰), respectively. δ¹³C values of P. euphratica leaves in different habitats were significantly different (P<0.05), and δ¹³C values of sapling leaves were all greater than those of mature trees. The average δ¹⁵N values of saplings and mature forest leaves were 3.259‰(-1.842‰-9.082‰) and 3.651‰(0.798‰ - 5.779‰), respectively. (2) The variation ranges of carbon content in leaves of saplings and mature forests were 44.573‰ - 49.056‰ and 43.226‰ - 47.349‰, respectively, with an average values of 46.225‰ and 45.720‰. The variation ranges of nitrogen content in young trees and mature forest leaves were 1.327‰ - 2.116‰ and 1.164 - 2.450‰, respectively, with the average values of 1.708‰ and 1.823‰. The content of carbon in mature forest leaves was negatively correlated with its δ¹³C value (P<0.01), while the content of nitrogen was positively correlated with δ¹³C value (P>0.05). There was a significant positive correlation between carbon content and δ¹⁵N value in mature forest leaves (P<0.01), while there was a significant positive correlation between nitrogen content and δ15N value (P<0.05). (3) The average specific leaf area value of saplings (91.565 cm²/g) was lower than that of mature trees (103.141 cm²/g). (4) δ¹³C and δ¹⁵N values in leaves of young and mature trees were significantly positively correlated with latitude (P<0.01), δ¹³C and δ¹⁵N values in leaves of young trees were also significantly positively correlated with altitude (P<0.01), and δ¹⁵N value in leaves of young trees was also significantly positively correlated with longitude (P<0.05). (5) The average water use efficiencies of young and mature trees were 77.618 μmol/mol (68.070 - 91.069 μmol/mol), 72.463 μmol/mol (62.809 - 97.111 μmol/mol), respectively. The water use efficiencies of young and mature trees were significantly positively correlated with their leaves δ¹³C (P<0.001), their δ13C values (-26.695‰ and -26.033‰) and water use efficiencies (91.069 and 97.111 μmol/mol) were the highest in Yutian County.