Populus simonii, which is one of the dominant and fastgrowing species in the Liudaogou watershed of China, plays a crucial role in the ecological restoration and construction of the area. However, there are few studies on the changes in photosynthetic parameters, water potential, δ¹³C, the total nitrogen and total phosphorus variations along the tree height in this area. Consequently, we measured these indicators of the leaves at different months and canopy leaves, and analyzed the water use efficiency and photosynthetic characteristics of P. simoniis leaves at different canopy heights. The results showed that: (1) the photosynthetic rate (P_n), stomatal conductance (G_s), intercellular CO₂ concentration (C_i) and transpiration rate (T_r) decreased with the increasing canopy height, indicating that the leaves were subjected to water stress caused by the tree height; (2) The values of δ¹³C and instantaneous water use efficiency (WUEi) increased with the increasing canopy height, and the δ¹³C values ranged from -29.81‰ to -27.43‰ along the canopy height; (3) The δ¹³C values of the leaves were positively correlated with the water use efficiency of the plant, and they could reflect the longterm water use status. The variation range of δ¹³C value was 2.22‰ and 2.12‰ along the tree height in different months, which suggested that genetic characteristics and environmental factors have common effects on leaf δ¹³C. This research provides a theoretical basis for elucidating the area of P. simonii mechanism of the tree height of water stress, and for the construction and management of arid and semi arid region of P. simonii plantation.