Abstract: Under the natural condition, the different intensities of photosynthetically active radiation (PAR) and different ranges of CO2 concentration, the characteristics of leaf photosynthesis and water ecophysiology of Populus adenopoda were studied to explore the response of photosynthesis, transpiration and water use efficiency to light intensity and elevated CO2 concentration by using twoyears old young trees as materials. The results were as following: (1) Under the condition of 400μmol/mol CO2, the regression equations between net photosynthesis rate (Pn) and PAR was yPn=-1.0×10-5x2PAR+0.030 3xPAR-1.103 1 with regression coefficient of 0.988 2; the light saturation point, light compensation point and apparent quantum yield (AQY) were 1 515 μmol/(m2·s), 55 μmol/(m2·s) and 0.045, respectively; the regression equation between transpiration rate (Tr) and PAR was yTr=0.002 6xPAR+7.153 2, which correlation coefficient is 0.878 6; and the regression equation between water using efficiency (WUE) and PAR was yWUE=-1.0×10-6x2PAR+0.003 5xPAR-0.093 3, and its correlation coefficient was 0949 3. (2) Under the condition of 1 000 μmol/(m2·s) PAR, the regression equation between Pn and CO2 concentration was yPn=2.0×10-5 x2CO2+0.047 2 xCO2-2.852 3 with r equaled to 0950 2, CO2 saturation point, CO2 compensation point and carboxylation efficiency were 1 180 μmol/mol, 64 μmol/mol and 0.058, respectively; However, Tr changed randomly with the increment of CO2 concentration, and there was no significant correlation between them; the regression equation between WUE and CO2 concentration was yWUE=3.0×10-6x2CO2+0007 2 xCO2-0.450 4 (r=0.926 3).