The degradation of forestland productivity in continuous-cropping poplar plantations is strongly related to the accumulation of phenolic acids in the soil. Therefore, the present study aimed to examine the effects of phenolic acid concentration on the photosynthetic characteristics and growth traits of poplar and to clarify the stress mechanism of phenolic acids on poplar growth. We established three treatments (CK, 0.5X, and 1.0X), based on the field concentration (X) of phenolic acids in the soil of a second-generation continuous-cropping poplar plantation and measured the physiological and biochemical parameters and growth indices of treated one-year-old black poplar cultivar I-107 (Populus×euramericana ‘Neva’). The results indicated that net photosynthetic rate (P_n), stomatal limitation (L_s), water use efficiency (WUE), transpiration rate (T_r), light-saturated net photosynthetic rate (P_nmax), apparent quantum yield (φ), dark respiration rate (R_d), maximal fluorescence yield of the dark-adapted state (F_m), effective quantum efficiency of PSⅡ (φ_PSⅡ), potential quantum efficiency of PSⅡ (F_v/F_m), photochemical quenching coefficient (qP), electron transport rate (ETR), chlorophyll (Chl) content, and total biomass decreased significantly with increasing concentrations of phenolic acids, whereas the intercellular CO₂ concentration (C_i), light compensation point (LCP), minimal fluorescence yield of the dark-adapted state (F_o), non-photochemical quenching (NPQ), malondialdehyde (MDA) content, and root/shoot ratio (R/S) increased significantly. We concluded that phenolic acid stress significantly reduces the photosynthesis, transpiration and respiration of poplar and that phenolic acid concentration and the inhibition of the poplar's physiological activities are positively correlated. With increasing phenolic acid concentrations, the organic matter accumulation of the poplar seedlings decreased significantly, and poplar leaf cells were seriously damaged. Therefore, poplar responds to phenolic acid stress by increasing the dissipation of excess light energy to alleviate damage to the photosynthetic apparatus under strong light conditions and by distributing photosynthates underground to promote root growth and nutrient absorption.