Abstract: Light response curve and diurnal change of photosynthesis characteristics were measured in different canopy layers of poplar clone 107 (Populus×euramericana cv. Neva), and the main ecophysiological factors affecting net photosynthesis rate were analyzed. The results showed that light response curve of different canopy layers of poplar clone 107 was simulated better with a new model of Pn=α(1-βI)(I-Icp)1+rI. The maximum net photosynthetic rate of the canopy Ⅰ, Ⅱ and Ⅲ displayed in light response curve were 10.076, 10.152 and 12.318μmol/(m2·s) respectively. And the diurnal average of net photosynthetic rate of the canopy displayed in an ascending order as: canopy Ⅰ,Ⅱ and Ⅲ. These results showed that the StrongWeak order of canopy photosynthetic capacity was the canopy Ⅲ, Ⅱ, Ⅰ. The contributions of main ecophysiological factors to net photosynthetic rate in different canopy layers were determined using Path Analysis. For canopy Ⅰand Ⅱ, the consistency of CO2 among cells and air temperature were the main factors to impact net photosynthetic rate. For the canopy Ⅲ, stomatal conductance, consistency of CO2 among cells, photosynthetic active radiation and air temperature were the main factors.