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This study explored the changes in the photosynthetic characteristics of the Fengdan peony under high-temperature stress to provide a reference for understanding the tolerance of peony plants under heat stress. In this study, the effects of high-temperature stress (40 °C) on the photosynthetic characteristics of the Fengdan leaves were studied. At 25 °C, the net photosynthetic rate (Pn), stomatal conductance (Gs), and transpiration rate (Tr) of the leaves decreased gradually with the increase in heat stress time, and intercellular CO₂ concentration (Ci) decreased first and then increased. High-temperature stress reduced the light energy absorption (ABS/RC) and capture (TR_O/RC), light energy for electron transport (ET_O/RC), and heat dissipation (DI_O/RC) per unit leaf area. The maximum photochemical efficiency (F_V/F_M), leaf photochemical performance index (PI_abs), the probability that captured excitons can transfer electrons to other electron acceptors in the electron transport chain beyond Q_A (ψ_O), and the quantum yield for electron transport (<i>φEo</i>), decreased gradually. The results showed that high temperatures damaged the photosynthetic capacity of the peony leaves and destroyed the photosynthetic apparatus of leaves. This study provides a reference for understanding the photosynthetic characteristics and tolerance of peony plants under heat stress.