In order to explore the effects of spermidine (Spd) on the growth and salt tolerance of oat seedlings, under 70 mmol/L salt (NaCl and Na₂SO₄ molar ratio 1∶1 mixed) stress, we used the saltsensitive variety ‘Baiyan5’ as the test material with hydroponics methods. In the present study, we investigated the effects of different concentrations of spraying spermidine solution on oat seeding growth, root activity, antioxidant enzyme activity, malondialdehyde, free proline and ion accumulation. The results showed that: (1) salt stress significantly inhibited the growth of oat seedlings. Compared with spraying distilled water under salt stress, spraying 0.75 mmol/L Spd significantly increased the aboveground dry weight, underground dry weight and root activity of seedlings by 34.1%, 23.8% and 24.7%, respectively. (2) The activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) increased by 17.0%, 22.9% and 23.7% in leaves and increased 43.0%, 19.4% and 91.2% in roots, respectively. MDA production in leaves and roots was inhibited by 25.2% and 12.8%, respectively. (3) It promoted the accumulation of free proline (Pro) in leaves and roots by 63.3% and 362.6%, respectively, and reduced the values of Na⁺/K⁺, Na⁺/Ca²⁺ and Na⁺/Mg²⁺ in leaves by 6.7%, 16.3% and 4.9%, respectively. our results indicate that foliar spraying of 0.75 mmol/L Spd can improve seedling antioxidant and osmotic adjustment ability under salt stress conditions, through maintaining the stability of the membrane system, reducing the damage of osmotic stress and ion poisoning to the seedlings, and enhancing the overall tolerant ability of the plants to salt stress.