The study aimed to investigate the effect of canopy and lateral branches on photosynthesis and leaf area of poplar trees of different ages In this paper poplar trees were taken as experimental objects and different canopy layers were divided according to three kinds of poplar trees of different ages 6 12 and 18 years old The primary lateral branches of the canopy were stratified from the outside to the inside to analyze photosynthetic characteristics leaf area and water use efficiency WUE and the differences in photosynthetic characteristics of leaves of different canopy layers and lateral branches of poplar trees of different ages were systematically studied The results showed as follows 1 The photosynthetic characteristics of the same age increased significantly with the increase of canopy and the lateral branches increased gradually from inside to outside P<0 01 2 In canopy 4 and 5 except for WUE value photosynthetic parame ters of 18-year-old were higher than those of 12-year-old P_n value was higher by 16 96% G_s value was higher by 54 50% and T_r value was higher by 32 52% In the canopy 3 the highest water utilization rate of 12-year-old was 4 678% and 25 447% respectively higher than that of 18-year-old and 6-year-old 3 There were significant differences in the photosynthetic parameters of leaves at different branches of the crown in the three kinds of age showing that the upper branch>the middle branch>the lower branch 4 Net photosynthetic rate was significantly positively correlated with G_s T_r and WUE P<0 01 and was significantly positively correlated with specific leaf area P<0 05