【Objective】 Populus simonii × P. nigra belongs to the genus Populus, family Salicaceae, and grows in northern China. This hybrid tree grows fast, reproduces easily, develops quickly, and has strong adaptability. It has been reported that P. simonii × P. nigra has biological characteristics that contribute to various types of resistance, such as to drought, barren conditions and alkali salt. This is an important fast-growing and high-yield timber species in China. Acid rain has negative impacts on the vegetation of ecological systems and is widespread in northern and southern China. However, relatively little information is known about the influence of acid rain on the growth and photosynthetic characteristics of P. simonii × P. nigra in China. To elucidate this issue, we studied the responses of P. simonii × P. nigra to simulated acid rain (SAR) at different pH values. The growth indices and leaf ultrastructure, antioxidant enzymes, and photosystem Ⅱ activities of P. simonii × P. nigra were studied in response to SAR stress. The results could provide necessary referenced data for planting P. simonii × P. nigra in northern China, where acid rain has increased. 【Method】 P. simonii × P. nigra stecklings were treated with SAR at three levels. The acid rain concentrate was diluted to simulate severe acid rain (pH 2.5) and moderate acid rain (pH 4.5), and diluted electrolyte liquor was used as a control (CK, pH 7.0). All treatment P. simonii × P. nigra stecklings were sprayed with SAR every day. To account for the stomatal distribution, leaves were sprayed with acid rain on both sides. Meanwhile,the pots were covered with a plastic hood to avoid the indirect effects of SAR from the soil. When apparent symptoms occurred in leaves, we detected and analyzed the effects of acid rain on the index of growth, leaf anatomic structure, physiological indices, OJIP curves, and chlorophyll fluorescence parameters; chlorophyll a fluorescence technology is considered a probe to monitor the photosynthetic characteristics of plants, which is crucial for analyzing the extent of damage to the photosynthetic apparatus under various stress conditions. 【Result】 Compared to CK, the plant height, stem diameter, leaf number and leaf area increased significantly in the SAR treatment at pH 4.5. The chloroplast structure was complete, and the thylakoids were intact, but the leaf color became light green, there were no starch grains, and the number of osmiophilic droplets increased slightly. Moreover, the stem diameter and leaf area in SAR at pH 2.5 were significantly reduced. The leaves became yellow, prominent spots appeared on the blade tips and veins, and there were a small number of starch grains, but the number and volume of osmiophilic droplets increased significantly. The physiological index, the rate of {\mathrm{O}}_2^{\stackrel{-}{\underset{·}{\mathrm{}}}}production, and the content of H₂O₂ and CAT activities were higher than that of CK, and the greater the acidity, the more significant the increase. However, POD and SOD activities were increased in the pH 4.5, but decreased in the pH 2.5. There was no noticeable difference in the treatment of pH 4.5 and CK except for relative variable fluorescence of point L at 0.15 ms (V_L), and relative variable fluorescence of point K at 0.3 ms (V_K). Correspondingly, the parameters of V_L and V_K and relative variable fluorescence of point J at 2 ms (V_J), relative variable fluorescence of point I at 30 ms (V_I), and the relative reduced rate of Q_A (M_O) increased significantly at pH 2.5 treatment. However, the chlorophyll fluorescence curve area (Fix Area) and the size of the plastid quinone pool (S_m) in the pH 2.5 treatment were significantly lower than those in the CK. 【Conclusion】 The effect of simulated acid rain on the growth of P. simonii × P. nigra mainly depends on the pH value. When SAR at pH 2.5 inhibited the growth and leaf physiology of P. simonii × P. nigra and showed symptoms of necrotic spots, the chloroplast structure was abnormal, the thylakoid arrangement was irregular, and the electron transfer of photosystem Ⅱ was inhibited. However, pH 4.5 SAR treatment benefited P. simonii × P. nigra, and the ultrastructure of leaves was normal. This indicates that P. simonii × P. nigra stecklings had a high tolerance to acid rain at pH 4.5.