Abstract: Hydrogen peroxide (H₂O₂) is a signaling molecule with diverse physiological functions in plants. The objective of this study was to determine whether exogenous H₂O₂ could improve the alkali tolerance of oat (Avena nuda) seedlings. Seedlings of oat cv. Dingyou No.6 at the three-leaf stage were irrigated with 75 mmol·L^-1 NaHCO₃ solution containing dimethylthiourea (DMTU, a quencher of H₂O₂) or ascorbate (ASA, a scavenger of H₂O₂), and the leaves were sprayed with 0.01 mmol·L^-1 H₂O₂. Seedlings irrigated with water served as the control. After 7 days of treatment, seedling growth, chlorophyll content, active oxygen metabolism, and osmotic accumulation in leaves were measured. The results showed that treatments with exogenous H₂O₂ remarkably relieved the growth inhibition caused by NaHCO₃ stress, and significantly decreased the contents of O2·-, H₂O₂, malondialdehyde, soluble sugars, free amino acids, proline, and activities of catalase and peroxidase. Treatments with exogenous H₂O₂ also increased the activities of superoxide dismutase and ascorbate peroxidase, and increased the contents of ascorbic acid, glutathione, organic acids, and chlorophyll in leaves of oat seedlings. However, none of the treatments affected the soluble protein content. The regulating effects of exogenous H₂O₂ on alkali stress could be reversed by treatment with DMTU or ASA. A comprehensive evaluation based on principal components and subordinate function analyses revealed that foliar spraying with H₂O₂ could improve the alkali stress adaptability of oat seedlings, and the promoting effect of H₂O₂ could be reversed by DMTU or ASA. Together, the results showed that H₂O₂ can increase alkali resistance in oat seedlings by regulating active oxygen metabolism and osmotic accumulation in leaves oat plants under alkali stress.