To understand the regulative effect of exogenous H₂O₂ on the physiological and biochemical responses of oat (Avena nuda L.) seedlings under salt stress, a new cultivar, ‘Dingyou No. 6’ seedlings grown in Hoagland’s nutrient solution was used to investigate the effects of exogenous H₂O₂ on active oxygen metabolism, osmoticum accumulation and homeostasis of Na⁺, K⁺ in leaves of oat seedlings under NaCl stress. Exposure to NaCl at 50-100 mmol·L^-1 did not have significantly unfavorable effect on plant growth, however, exposure to 150 mmol·L^-1 and above of NaCl caused reductions in the plant dry weight and K⁺/Na⁺ ratio, and increases in the O₂^-_·  generation rate, contents of H₂O₂, malondialdehyde (MDA), soluble protein and praline, and activities of catalase (CAT) and plasma membrane H⁺ATPase in the seedling leaves, but no detectable changes in ascorbic acid (ASA) and glutathione (GSH) contents. The addition of 5 μmol·L^-1 H₂O₂ significantly alleviated the inhibitory effect of 150 mmol·L^-1 NaCl stress on the growth of oat seedlings by increasing the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and CAT and the contents of GSH and H₂O₂, and decreased O₂^-_·  generation rate and MDA content in the seedling leaves. In addition, exogenous H₂O₂ supply also increased the contents of soluble protein, soluble sugar, organic acid and proline, plasma membrane H⁺ATPase activity and K⁺/Na⁺ ratio, while decreased free amino acid content in plant leaves under 150 mmol·L^-1 NaCl stress. These results indicated that exogenous H₂O₂ could enhance the tolerance of oat seedlings to salt stress by modulating antioxidative defense system and osmoticum accumulation, and maintaining K⁺/Na⁺ ratio in plant leaves.