In the present study, we investigated the physiological mechanism of foliar spraying of exogenous melatonin (MT) on maize under drought stress and rewatering. The maize cultivar ‘Shaanke No.9’ applied with 100 μmol·L^-1 melatonin was used as the experimental materials, the changes of leaf relative water content (RWC), leaf area, aboveground biomass, photosynthetic characteristics and antioxidant enzyme activities were determined under severe drought stress and rewatering. The results suggested that: (1) foliar application of exogenous melatonin could alleviate the maize growth inhibited by drought stress. It also improved the effective quantum yield of PSⅡ and PSⅠ under drought stress ［Y(Ⅱ), Y(Ⅰ)］, decreased the PSⅠ receptor side limitation ［Y(NA)］ and donor side limitation ［Y(ND)］ of leaf photosynthetic apparatus after drought stress. (2) The melatonintreated plants exhibited lower malondialdehyde (MDA) content and hydrogen peroxide (H₂O₂) content. The activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) were enhanced and the expression of antioxidant enzymes related genes were upregulated by melatonin treatment under drought stress. (3) After rehydration, compared with untreated maize, higher recovery rates of parameters were obverted in exogenous melatonintreated maize leaves. Consequently, foliar spraying of melatonin effectively alleviated the damage to photosynthetic apparatus of maize leaves under drought stress, enhanced the activities of antioxidant enzymes and the expression of related genes, significantly reduced the level of membrane lipid peroxidation, and substantially promoted the recovery of physiological functions of maize leaves after rehydration. In short, exogenous melatonin can improve the photosynthetic efficiency and antioxidant capacity of maize leaves under drought and recovery water, eventually promoting the plant growth to adapt to the stress environments.