In order to explore the effects of exogenous nitric oxide (NO) on the salt tolerance of melon seedlings under 300 mmol·L^-1 NaCl stress,we used melon variety ‘Nongda Tian No. 10’ as the tested material. In the present study, with different concentrations of spraying exogenous NO donor sodium nitroprusside (SNP) (50, 100, 150, 200, 250 μmol·L^-1), we investigated the changes of melon seedlings growth, photosynthetic pigment content, cell membrane permeability, antioxidant enzyme activities under 300 mmol·^-1 NaCl stress. The results showed that: (1) salt stress significantly inhibited the growth of melon seedlings, and significantly reduced the content of photosynthetic pigment, cell membrane permeability, antioxidant enzyme activities and osmoregulation substances in leaves. (2) Foliar spraying 150 μmol·L^-1 SNP significantly improved the plant height, stem diameter, dry and fresh weight and strong seedling index of melon seedlings under salt stress, significantly increased the photosynthetic pigment content of melon seedlings under salt stress, improving the photosynthesis of melon. (3) Spraying 150 μmol·L^-1 SNP significantly increased the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbic acid peroxidase (APX), glutathione reductase (GR), monodehydroascorbic acid reductase (MDHAR), dehydroascorbic acid reductase (DHAR) in melon seedlings under salt stress, so as to significantly reduce malondialdehyde (MDA) and Hydrogen peroxide (H₂O₂) contents and superoxide anion ( O₂^-·) production rate. (4) Spraying 150 μmol·L^-1 SNP also decreased the contents of proline (Pro) and soluble protein in the leaves of melon seedlings under salt stress. Therefore, under salt stress environment, the appropriate concentration of exogenous SNP (150 μmol·L^-1) can enhance the scavenging capacity of reactive oxygen species, reduce membrane lipid peroxidation and effectively reduce the damage of salt stress to seedlings by increasing the activities of antioxidant enzymes and the contents of photosynthetic pigments and small molecular soluble organic compounds in melon seedlings, so as to enhance their salt tolerance and promote the growth of seedlings.