Alfalfa, the perennial legume forage, soil salinization is becoming a yield limiting factor of it. ‘Zhongmu No.1’, as one of the Alfalfa, was used as the material in this study. Based on the screening test of salt stress and melatonin, salt (150 mmol/L NaCl) and melatonin (30, 50, 80 μmol/L MT) separate and combination treatment were selected. The effects of exogenous melatonin on the growth characteristics, membrane permeability, osmotic regulation and effects of antioxidants enzymes and photosynthetic indexes were shown in this article. The results showed that: (1) the growth of alfalfa seedlings was significantly inhibited by salt stress, but not affected by various concentrations of melatonin. All melatonin treatments effectively alleviated the damage caused by salt stress to the growth of alfalfa, and the effect of 150 mmol/L NaCl+ 80 μmol/L melatonin treatment (NaCl + MT2) was the best, the seedling foot length, the root fresh weight and root dry weight increased significantly by 34.52%, 41.93% and 19.61% compared with salt stress treatment, respectively. (2) Salt stress significantly increased the permeability of cell membrane system and the degree of membrane lipid peroxidation of alfalfa seedlings. The relative conductivity and MDA content of seedling leaves treated with NaCl + MT2 were significantly lower than those treated with salt stress by 27.18% and 30.24%, respectively. At the same time, the relative water content in seedling leaves was significantly increased, but the proline content was significantly reduced, exogenous melatonin effectively alleviated the damage of cell water loss and cell membrane under salt stress. (3) The activities of POD and SOD in the leaves of NaCl + MT2 seedlings were significantly higher than those of salt stress treatment by 31.45% and 41.41%, respectively. However, the CAT activities had no significant change. Exogenous melatonin could significantly enhance the activities of POD and SOD in the leaves of alfalfa seedlings, improve the scavenging ability of reactive oxygen species and reduce the peroxidation damage induced by salt stress. (4) Salt stress significantly inhibited the photosynthetic efficiency of alfalfa seedlings, while the net photosynthetic rate, stomatal conductance and transpiration rate of seedlings treated with NaCl + MT2 increased by 30.27%, 45.1% and 42.15%, respectively compared with salt stress. It was found that salt stress significantly increased the accumulation of reactive oxygen species in alfalfa seedling leaves, reduced the activities of antioxidant enzymes, and reduce its photosynthetic efficiency. Exogenous melatonin promoted the growth of alfalfa seedlings by improving its antioxidant capacity and photosynthetic efficiency, so our study provides a theoretical basis for improving alfalfa salt tolerance and reforming saline alkali land.