A pot experiment was conducted to study the effect of exogenous spermidine (Spd) on the seed germination and seedling antioxidant system of tomato cultivar 'Baiguoqiangfeng' under NaCl stress. The results showed that after soaking with Spd, the tomato seed's germination rate (Gr), germination index (Gi), and vigor index (Vi) increased by 104.90%, 142.31% and 122.22%, respectively, and the mean germination time (MGT) shortened by 4.97%, compared with the control. Soaking seed with Spd increased the seedling leaf's superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities and soluble protein content, and decreased its superoxide anion (O2) production rate, hydrogen peroxide (H2O2) content, malondialdehyde (MDA) content, and electrolyte leakage of plasma membrane. The seedling growth was also promoted. It was concluded that soaking seed with Spd could promote the seed germination and seedling growth of tomato under NaCl stress, and enhance the salt-resistance of tomato plant.