Abstract: 【 Objective 】 Soil salinization seriously affects the growth of cotton seedlings, and Melatonin (MT) plays an important role in regulating the salt tolerance of plants. In this study, the method of spraying exogenous melatonin on the leaf surface of cotton seedlings was used to explore the response of exogenous melatonin to cotton seedling growth and photosynthetic system under NaCl stress, and to screen out the most suitable spraying melatonin concentration. Effects of Exogenous Melatonin on Growth and Photosynthetic Characteristics of Cotton Seedlings under Salt Stress. 【 Methods 】 Xinluzhong 70 was taken as the material, and different melatonin concentrations (25, 50, 100, 200 μmol/L) treatment were set tp measure cotton seedling growth indexes, leaf fluorescence, photosynthesis parameters and other indexes in different time periods (1, 3, 6, 9 d) under NaCl stress. 【 Results 】 NaCl stress significantly reduced plant height, leaf photosynthesis and fluorescence indexes of cotton seedlings after spraying melatonin and the plant height and dry matter quality of cotton seedlings were increased. Furthermore, the effects of NaCl stress on the net photosynthetic rate, stomatal conductance, maximum photochemical efficiency, PSII effective photon yield, photochemical quenching and other indicators of cotton seedling leaves were effectively alleviated and the damage of NaCl stress to cotton seedlings was also reduced. 【 Conclusion 】 The results showed that under NaCl stress spraying exogenous melatonin can effectively promote the growth of cotton seedlings, and spraying 100 μmol/L melatonin concentration has the best result.