The oneyearold seedlings of Paeonia rockii were used as test materials, and different concentrations (0, 100, 300, 500 mg/L) of gibberellin (GA₃) were sprayed on the leaves, and the anatomical structure of the seedling leaves was observed by transmission electron microscope, scanning electron microscope and optical microscope. Photosynthetic instrument was used to measure the photosynthetic parameters of the seedlings, and the enzymelinked immunosorbent method was used to measure the endogenous hormone content in the leaves, to explore the effect of exogenous GA₃ on the anatomical structure, photosynthetic characteristics and endogenous hormone levels of the seedlings of P. rockii. The results showed that: (1) the mesophyll cells of P. rockii treated with low concentration GA₃ increased, and the number of chloroplasts in the outer cells of the fence tissue increased, while the high concentration GA₃ treatment was the opposite; the ratio of fence tissue/sponge tissue (P/S), tissue structure compactness (CTR) decreased, and its tissue structure porosity (SR) value increased; the size of each chloroplast in the mesophyll cells of the seedling leaves treated with GA₃ was significantly larger than that of the control. As the concentration of GA₃ treatment increased, The volume of chloroplasts in the mesophyll cells tends to increase, the stacks of thylakoids are gradually loosened, and the starch granules on the chloroplasts are more obvious in the 300 mg/L GA₃ treatment; the length, width, stomatal apparatus size, stomata opening of leaves and stomata density increased at first and then decreased with the increase of GA₃ concentration, and the thickness of the upper epidermal stratum corneum of the leaves increased with the increase of GA₃ concentration. (2) The net photosynthetic rate (P_n), stomatal conductance (Cond), transpiration rate (T_r), and water use efficiency (WUE) in leaves of P. rockii were significantly higher than those of the control under 100 and 300 mg/L GA₃ treatments. The 300 mg/L GA₃ treatment was significantly higher than the other treatments, while it was significantly lower than the control under the 500 mg/L GA₃ treatment. (3) The contents of abscisic acid (ABA) and indole acetic acid (IAA) in the leaves of P. rockii was significantly higher than that of the control at 500 mg/L GA₃, but was lower than the control to varying degrees under other treatments. The endogenous zeatin riboside (ZR) and GA₃ contents in the leaves were significantly higher than the other treatments and the control under the 300 mg/L GA₃ treatment, while the other treatments had no significant changes compared with the control; The leaves ZR/ABA, ZR/IAA, ZR/GA₃ and (IAA +GA₃+ZR)/ABA ratios were significantly higher than other treatments under 300 mg/L GA₃ treatment, and the IAA/ABA and ABA/GA₃ ratios of the leaves were significantly higher than other treatments under 500 mg/L GA₃ treatment. The study found that treatment with an appropriate concentration of exogenous GA₃ can significantly increase the leaf photosynthetic rate, water use efficiency and transpiration rate of seedlings of peony, adjust the contents and balance of endogenous hormones in the plant, so that the leaves can synthesize more organic matter and promote the growth of seedlings.