Through the outdoor potted plant trials, the allelopathic potential of Populus tomentosa was tested against its species in the growth, chlorophyll content, and photosynthetic and chlorophyll fluorescence characteristics with aqueous extracts (0.01,0.02,0.05, and 0.1 g · mL-1) obtained from leaves at different individual ages (1, 20, and 45 years old). The results showed that seedling height, basal diameter, fresh and dry weights, quantity of chlorophyll, the ratio of chlorophyll a/b, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), efficiency of primary conversion of light energy of PSII (Fv/Fm), potential activity of PSII (Fv/Fo), and photochemical quenching (qP) of the seedlings gradually decreased with the increase of extract concentration of all three ages when compared with the controls. The older the P. tomentosa used for extract preparation, the greater the percentage declined in the aforementioned parameters. Moreover, at the four concentrations used, there was a significant difference between treatments with the extracts from 1- and 45-year-old plants (except for qP), but occasionally, the effects were not obvious between the 1-and the 20-year-old plants, or the 20- and 45-year-old plants. The intercellular CO2 concentration (Ci) treated with the extracts from the 1-year-old decreased at the lowest concentration, whereas it increased at higher concentrations. The C, treated with aqueous leaf extracts from the 20-year-old decreased at the lower concentrations and increased to similar levels to that of the control at the higher concentrations. C, was always close to control levels in 45-year-old extract treatments. All the aqueous leaf extracts of P. tomentosa at all ages caused an increase of the initial fluorescence (Fo). The older P. tomentosa used for the preparation of aqueous leaf extracts caused a greater percentage decline in Fo. The nonphotochemical quenching (qN) increased significantly at lower concentrations of all P. tomentosa extracts, whereas it decreased significantly at higher concentrations. It seemed that aqueous leaf extracts from P. tomentosa were harmful to the photosynthetic structure of its own seedlings, inhibited seedling growth, and led to an eventual decrease of biomass. Extracts from older P. tomentosa leaves had more negative effects on the seedling growth of poplar. The effects on photosynthesis are the more important mechanism of the allelopathy of poplar.