Abstract: Taking tomato ‘Riegle 87-5’ as test materials,the effects of spraying different concentrations of melatonin(50-500μmol·L^-1)on the growth and physiological indexes of tomato seedlings under NaHCO₃stress,tomato seedlings were processed by hydroponic method and simulated 60mmol·L^-1NaHCO₃ environment,in order to provide reference for using MT to solve alkali stress in tomato production.The results showed that compared with CK,MT₀ significantly reduced the plant height and fresh and dry weight of aboveground and underground parts of seedlings,MT₂₀₀ significantly alleviated the morphological indexes of plant growth and development,and increased the plant height,fresh weight of aboveground and fresh dry weight of underground parts by 31.57%,73.86%,1.71 and 1.94 times.Compared with CK,Pn,Gs,Ci and Tr decreased significantly under MT₀ treatment,and photosynthetic characteristics alleviated most significantly under MT₄₀₀ treatment.In terms of osmotic stress,MT₀ significantly increased the relative electrical conductivity （REC）,malondialdehyde（MDA）,hydrogen peroxide（H₂O₂）and superoxide anion■production rates,significantly reduced the activities of SOD and POD in tomato leaves,and stimulated the activities of CAT and APX enzymes compared with CK.MT treatment,especially MT₄₀₀ could significantly increase the activities of SOD and POD,and decrease REC,MDA,H₂O₂,■．In conclusion,MT in a certain concentration range could improve the growth morphology of tomato seedlings under alkali stress(60mmol·L^-1),enhance photosynthesis,improve the activity of antioxidant enzymes,reduce cell membrane lipid peroxidation,and ultimately improve the alkali tolerance of seedlings.