In order to explore the differences in physiological responses of oat(Avena nuda L.) seedlings to NaCl and KCl stress, the changes in seedling growth, reactive oxygen species metabolism and osmotic adjustment in leaves of oat seedlings under NaCl and KCl stress of different concentration were investigated by hydroponics method. The results showed that:(1) At the concentration of 75 mmol/L and 150 mmol/L, NaCl stress had greater inhibition effect on the seedling growth and greater damage to the membrane lipid peroxidation than that of KCl stress, the activities of superoxide dismutase(SOD) and catalase(CAT) and the contents of soluble protein, soluble sugar and proline in the seedling leaves of NaCl stress were lower than that under KCl stress; The contents of O₂^-., H₂O₂, malondialdehyde(MDA), soluble protein and soluble sugar under KCl stress were much higher than that under NaCl stress when the concentration increase to 225 mmol/L, but the activities of SOD and ascorbate peroxidase(APX) and the glutathione(GSH) content of KCl stress were lower than NaCl stress.(2) After treated with 225 mmol/L of KCl and NaCl, the leaf water potential was-0.867 MPa and-1.034 MPa. Besides, the osmotic potential was-1.409 MPa and-1.252 MPa, respectively, indicating that the increased damage caused by 225 mmol/L of KCl was not due to the osmotic stress. Under the stress of 225 mmol/L KCl, the leaf Na⁺ content decreased by 36.5% and K⁺ content increased by 1.49 times, compared with the control, while the supplement of 20 mmol/L NaCl could increase the Na⁺ content and the activities of SOD and APX, decrease the contents of K⁺, H₂O₂, O₂^-.and MDA in leaves of oat seedlings under 225 mmol/L KCl stress significantly. The active oxygen accumulation induced by ion toxicity in oat seedling leaves could be responsible for the enhanced damage caused by 225 mmol/L of KCl than that by NaCl.