In order to explore the effects of exogenous melatonin on photosynthetic apparatus of maize leaves under drought stress and rewatering and clarify the physiological mechanisms involved, the biomass, net photosynthetic rate (P_n), and photosynthetic electron transfer rate in seedlings of maize shaandan609 with or without 100 μmol/L melatonin application were analyzed after severe drought and rewatering.. The results showed that exogenous melatonin alleviated growth inhibition caused by drought stress. Further investigation demonstrated that, under drought condition, melatonin-treated plants indicated higher P_n and stomatal conductance (G_s), as well as lower intercellular CO₂ concentration (C_i) than those untreated plants. In addition, exogenous melatonin increased the fluorescence parameters φ_P0, φ_E0, ψ₀ and photosynthetic performance index PI_abs of OJIP curve under drought stress. It also improved the effective quantum yield of PSⅡ and PSⅡ under drought stress [Y(Ⅱ), Y(I)]. The melatonin treatment decreased the PSI receptor side limitation [Y(NA)] and donor side limitation [Y(ND)] of leaf photosynthetic apparatus after drought stress. Moreover, exogenous melatonin increased the electron transfer rate (ETRⅡ, ETRI) of PSⅡ and PSⅡ in maize leaves under drought stress, indicating that exogenous melatonin could alleviate photo-inhibition under drought stress. After rewatering, all the parameters of maize leaves treated with melatonin returned to the control level under drought stress, while the parameters of maize leaves treated with drought stress could not recover after rehydration. It can be concluded that exogenous spraying melatonin can not only alleviate the damage of PSⅡ and PSI structure and function of maize caused by drought stress, but also accelerate the recovery of photosynthetic apparatus function after rewatering, resulting in improving the recovery of maize seedling growth. Therefore, exogenous melatonin application could be an important regulation mode to adapt to arid environment by enhancing photosynthesis of maize leaves.