Abstract: Salix purpurea, a native tree species in the northwestern China, grows fast and has strong adaptability. It plays important roles in water and soil conservation, shelter forest and bioenergy. In order to illustrate the photosynthetic mechanism and growth response of S. purpurea to soil drought stress, we studied the diurnal photosynthetic characteristics of the potted cutting plants under four different soil drougth stress(control, mild, moderate and severe drought stress). The results showed that the diurnal means of the net photosynthetic rates(P_n)reduced by 10.4%(P＞0.05), 17.9%(P＜0.05)and 58.2%(P＜0.05)under the mild, moderate and severe drought stress compared with control, respectively. And the P_n under moderate and severe drought stress reduced significantly. With the drought stress level increasing, the daily average of the net photosynthetic rates(P_n), transpiration rates(T_r), stomatal conductance(G_s)also decreased significantly, while water utilization efficiency(E_WUE)increased slightly. Unlike the above parameters, the intercellular CO₂ concentration(C_i)decreased initially and then increased. These results suggested that the decrease of P_n under mild and moderate drought stress might be caused by stomatal limitation, while the decrease under severe drought stress might be caused by stomatal and non-stomatal limitation. Furthermore, the relative growth ratio of height and ground diameter of S. purpurea cutting plants decreased significantly with the increased drought stress level.