Four irrigation treatments, CK(no irrigation), T1(20 kg/plant-session), T2(40 kg/plant-session) and T3(60 kg/plant-session), were used for 3, 5, 7 a poplar as the test material. The photosynthetic CO2 response processes of 3, 5, 7 a poplar to different water irrigation were measured by Li-6400 portable photosynthesis system to obtain CO2 characteristic parameters, and four light response models including right-angle hyperbolic model, non-right-angle hyperbolic model, right-angle hyperbolic modified model, and exponential model were used to fit and analyze the photosynthetic characteristic parameters. The results showed that the fitted coefficients of determination(R2) of each model were greater than 0.97. The right-angle hyperbolic modified model(MRHM) and the exponential model fitted the photosynthetic parameters of poplar better, and the MRHM was more accurate, with the total integrated average relative error(ARE) of 0.093, 0.084, 0.084 for the 3, 5, 7 a poplar eigenvalues under the three irrigation treatments, respectively. The net photosynthetic rate of 3 a poplar varied little under the three irrigation treatments, and the net photosynthetic rate of 5 a and 7 a poplar was maximum under 60 kg/plant-session treatment. In the comprehensive analysis, the optiaml irrigation amount for 3 a poplar growth was 40 kg/plan-session, and the optimal irrigation amount for 5 a and 7 a poplar growth was 60 kg/plant-session under different irrigation treatments.