Abstract: The experiment was conducted to study whether exogenous hydrogen peroxide(H₂O₂) could improve the saline-alkali tolerance of naked oat(Avena nuda) seedlings and its physiological mechanism. Seedlings of naked oat(‘Dingyou No.6’) at three-leaf period cultured with perlite in a solar greenhouse were irrigated with the Hoagland nutrient solution containing 75 mmol·L^-1 saline-alkali mixed solution(NaCl∶Na₂SO₄∶NaHCO₃∶Na₂CO₃=12∶8∶9∶1) combination with dimethylthiourea(DMTU, a quencher of H₂O₂), and the leaves were sprayed with 0.01 mmol·L^-1 H₂O₂. Seedlings were irrigated with Hoagland nutrient solution, and the leaves were sprayed with water served as the control. Seedlings growth, photosynthetic pigment content, active oxygen metabolism, and osmotic regulatory substance accumulation in leaves were measured. The results showed that exogenous H₂O₂ could alleviate the inhibition effects of saline-alkali mixed stress on seedlings growth of naked oat. Spraying H₂O₂ significantly increased root length, plant height, dry weight, the leaf chlorophyll a, chlorophyll b, total chlorophyll and carotenoid contents, and the superoxide dismutase, peroxidase, catalase, ascorbate peroxidase activities of naked oat seedlings under saline-alkali mixed stress, but decreased the contents of superoxide anion, H₂O₂, malondialdehyde, ascorbic acid, glutathione, and free amino acids. Treatments with spraying H₂O₂ also increased the contents of antioxidants(such as flavonoids, total phenols, procyanidins) and osmotic regulatory substance(such as soluble protein, soluble sugars, and proline) in leaves. The above effects of exogenous H₂O₂ were partially or completely reversed by DMTU. A comprehensive evaluation based on subordinate function analyses revealed that foliar spraying with H₂O₂ could improve the saline-alkali resistance of naked oat seedlings, and the promoting effect of H₂O₂ could be completely reversed by DMTU. Therefore, the exogenous H₂O₂ supply can increase saline-alkali adaptability in naked oat seedlings by regulating active oxygen metabolism and osmotic regulatory substance accumulation in leaves of naked oat plants under saline-alkali mixed stress.