【Objective】This study examined the effect and mechanism by which nano-sized iron oxide regulates crop tolerance to salt. 【Method】Nanoparticle Fe3O4 (Fe3O4NPs) with a particle size of about 10 nm was synthesized by the coprecipitation method and characterized by X-ray diffractometry and electron microscopy. Selected tomato seeds were soaked in 0, 1, 10, 50, 100, 200, 300, and 400 mg?L–1 of Fe3O4NPs solution and the effects on seed germination, seedling growth and antioxidant system under NaCl stress were evaluated. 【Result】 Fe3O4NPs with a particle size of 10 nm were synthesized and successfully passed the characterization analysis and identification. It was found that under salt stress, soaking tomato seeds in 1 mg?L–1 Fe3O4NPs reduced seed germination. However, seed germination was improved as the concentration of Fe3O4NPs was increased. Under 100 mmol?L–1 NaCl stress, the seed germination potential and hypocotyl length after soaking in 200 mg?L–1 Fe3O4NPs reached the peak, which was significantly higher than those treated with salt stress only. Salt stress of 100 mmol?L–1 NaCl significantly reduced the seedling rate, fresh biomass and water content of tomato seedlings, and this negative trend was intensified after soaking the seeds in 1 mg?L–1 Fe3O4NPs. With an increase in the concentration of Fe3O4NPs, the negative impact of salt stress was alleviated and the fresh biomass and water content of seedlings soaked with 200 mg?L–1 Fe3O4NPs reached the peak, which was significantly higher than those treated only with salt. Under salt stress, the activities of superoxide dismutase (SOD) and peroxidase (POD) in tomato seedlings soaked in 1 mg?L–1 Fe3O4NPs increased significantly, while the activity of catalase (CAT) decreased significantly. With an increase in the concentration of Fe3O4NPs, the activities of SOD and POD gradually decreased and then gradually increased while that of CAT gradually increased and then decreased. Specifically, the activities of SOD and POD in seedlings treated with 100-200 mg?L–1 Fe3O4NPs were the lowest, while the activity of CAT was the highest. Also, the contents of malondialdehyde (MDA), proline (Pro), superoxide anion (O2?－) and hydrogen peroxide (H2O2) in seedlings treated with 200 mg?L–1 Fe3O4NPs were the lowest of all treatments. The correlation analysis showed that the fresh biomass and seedling rate of seedlings were significantly negatively correlated with the activities of SOD and POD, as well as the contents of MDA and reactive oxygen species. 【Conclusion】 Fe3O4NPs with a particle size of 10 nm were prepared by the coprecipitation method. It was found for the first time that the germination and emergence of tomato seedlings under salt stress could be mitigated by Fe3O4NPs and the effect was proportional to the concentrations of Fe3O4NPs. For example, treating seeds with 1 mg?L–1 Fe3O4NPs inhibited germination due to increased oxidative stress whereas 200 mg?L–1 Fe3O4NPs showed significant promotion of germination, seedling formation and seedling strength. This study provides scientific basis and technical support for the application of nanomaterials to improve agriculture in saline soil.