Abstract: In order to explore the alleviating effect of exogenous melatonin on alkali stress of Setaria italica var. germanica, five kinds of Setaria italica var. germanica with different alkaline tolerance were used as materials to study the effects of different concentrations of melatonin（50, 100, 200, 300, 400 μmol/L） on the germination, growth and physiological characteristics of Setaria italica var. germanica under 25 mmol/L NaHCO₃ and Na₂CO₃（mixed molar ratio of 1 to 1） stress. The results showed that alkali stress significantly inhibited the germination and seedling growth of Setaria italica var. germanica, and different concentrations of exogenous melatonin could alleviate the inhibitory effect of alkali stress on Setaria italica var. germanica growth, among which 50 μmol/L melatonin was more effective on alkali-sensitive variety Jindu1, while the antioxidant enzyme activity of medium variety Changnong35 was more obvious, indicating that under alkali stress, melatonin could increase the activity of antioxidant enzymes, remove excess reactive oxygen species in cells, maintain cell stability, and improve the alkali tolerance of Setaria italica var. germanica.