Abstract: Soil salinity is a major limiting factor for plant growth and productivity globally. Hydrogen peroxide (H₂O₂) is an important signaling molecule in plants that regulates many important physiological and biochemical processes and induces tolerance to different stresses, including salt stress. A study has been undertaken in order to further understand the operation of these regulatory mechanisms in oat seedlings (Avena nuda). A new oat cultivar, ‘Dingyou No. 6’, was selected to investigate, using greenhouse nutrient solution cultivation, the effects of exogenous H₂O₂ on plant growth, osmotic adjustment substances accumulation and active oxygen metabolism in seedlings under salt stress. The results showed that 150 mmol/L NaCl exposure significantly inhibited seedling growth. It enhanced the production of free amino acid and proline and decreased the contents of glutathione (GSH) and soluble sugar in leaves. Foliar spraying of 0.010 mmol/L H₂O₂ significantly alleviated the inhibitory effect of NaCl stress on seedling growth. Exogenous H₂O₂ increased the contents of soluble protein, soluble sugar and proline, and decreased free amino acid content in leaves. Under 150 mmol/L NaCl stress, superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) activities all increased, along with excessive production of O2·-, H₂O₂ and malondialdehyde (MDA) in seedling leaves. Spraying the stressed seedlings with 0.010 mmol/L H₂O₂ treatments significantly increased the activities of SOD, CAT, POD and APX and GSH content, but decreased O2·- production rate and the contents of H₂O₂ and MDA in leaves. These results indicate that exogenous H₂O₂ could enhance anti-oxidative ability and decrease membrane lipid peroxidation injury in oat seedlings under NaCl stress. Exogenous H₂O₂ enhanced seedlings’ salinity tolerance by regulating osmotic adjustment substances accumulation and active oxygen metabolism in plant leaves.