Abstract: A pot experiment was conducted to study the effects of different water treatments (normal irrigation, light drought, moderate drought, and severe drought) on the growth, gas exchange, chlorophyll fluorescence characteristics, and active oxygen metabolism of poplar (Populus× euramericana cv. ‘Neva’) seedlings in the experimental nursery of Shandong Forestry Academy from April to October, 2011. As compared with those under normal irrigation, the growth of the seedling’s basal diameter under light, moderate, and severe drought stress decreased by 12.8%, 44.5%, and 65.6%, and the height growth decreased by 12.2%, 43.1%, and 57.2%, respectively. With the increasing extent and duration of drought stress, the maximal photochemical efficiency of PSⅡ, quantum yield, photochemical quenching coefficient, net photosynthetic rate, and stomatal conductance of the seedling leaves decreased gradually under light drought stress, while decreased rapidly under both moderate and severe drought stress. The nonphotochemical quenching coefficient increased significantly under light drought stress, but decreased after an initial increase under moderate and severe drought. The leaf superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities under drought stress decreased after an initial increase, but definite differences existed in the responses of the three enzymes to drought stress and reactive oxygen. The leaf relative electric conductivity and malondialdehyde (MDA) content under drought stress increased significantly, plasma membrane was damaged, and massive ions leaked out. The most serious damage of plasma membrane was found under severe stress. Under light drought stress, the seedlings had higher photosynthetic efficiency and stronger oxidative enzyme defense system; under moderate and severe drought stress, the photosynthetic efficiency decreased significantly, and the oxidative enzyme defense system was damaged remarkably.