Salt stress imposes a major environmental threat to agriculture; therefore, efforts to increase salt tolerance of crop plants bear remarkable importance for sustainable agriculture on saline lands and could potentially improve crop yield overall. Pakchoi (Brassica chinensis L.) seeds were pre-treated with 5, 10, 25, 50 and 100 mmol/L hydrogen peroxide (H₂O₂) solution for 2 h to evaluate the effect of H₂O₂ on germination, early growth, and physiological characteristics of seedlings under 100 mmol/L NaCl stress. Results showed that 100 mmol/L NaCl stress inhibited pakchoi seed germination and seeding growth. Germination potential, germination index, vigour index, and root length were decreased significantly. The osmosis-regulating substance, reactive oxygen species (ROS), and malondialdehyde (MDA) content in roots and shoots of pakchoi was increased significantly. However, the activities of catalase (CAT) and K⁺ content in roots and shoots were strongly inhibited. Different concentrations of H₂O₂ (5, 10, 25, 50, and 100 mmol/L H₂O₂) increased the seed germination potential, germination index, and vigour index, and promoted the growth of pakchoi roots and shoots. Application of H₂O₂ also significantly enhanced the activities of antioxidant enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), and CAT, and decreased the contents of MDA and reactive oxygen species like H₂O₂ and O₂^-· in roots and shoots of pakchoi seedlings, and further increased the proline and soluble sugar contents. In addition, K⁺ content and K⁺/Na⁺ in roots and shoots of pakchoi seedlings were increased by H₂O₂; however, Na⁺ content was decreased. 10 mmol/L H₂O₂ showed the best effect for improving seed germination and seeding growth of pakchoi under NaCl stress. However, the mitigative effects of 100 mmol/L H₂O₂ on inhibition of pakchoi seed germination and seedling growth caused by NaCl stress were weakened. Hence, an appropriate concentration of H₂O₂ treatment improved NaCl tolerance of pakchoi plants by increasing the plant's antioxidative defence system, the capacity for osmotic adjustment, and K⁺ content and decreasing the contents of ROS and Na⁺ to alleviate membrane lipid peroxidation, thereby increasing seed germination and seedling growth under NaCl stress.