The work aims to prepareChitosan nano-selenium (CS-SeNPs) and apply it in the cold storage of South Bay fish to prolong its shelf life. CS-SeNPs was prepared by chemical reduction of sodium selenite with ascorbic acid using chitosan as stabilizer and dispersant. A single factor test was conducted with the concentration of chitosan, the molar ratio of sodium selenite to ascorbate (Vc/Na2SeO3), reaction temperature and reaction time as factors. On the basis of the test, the stability of CS-SeNPs was taken as the response value, and the concentration of chitosan, Vc/Na2SeO3 and reaction temperature were selected as the objects of investigation. The optimum preparation process of CS-SeNPs was obtained by response surface optimization test. CS-SeNPs was applied to the preservation of South Bay fish and its preservation effect was investigated. The optimum process parameters of CS-SeNPs were as follows:the concentration of chitosan was 20 g/L, the reaction temperature was 40 ℃, the molar ratio of Vc/Na2SeO3 was 4:1, and the A410/A490 was 2.176±0.008 6. Compared with the control group, CS-SeNPs after treatment had a better fresh-keeping effect, which could delay the increase of lipid oxidation, the total number of colonies, the total volatile base nitrogen value and the SAP loss rate during fish storage, maintain the pH value of fish and extend the shelf life of fish in cold storage. The correlation analysis showed that the value of TVB-N and the rate of juice loss were highly correlated with the storage quality of frozen fish. Compared with the control group, the refrigerated shelf life of South Bay fish can be extended by 3-6 days in the treatment group, which provides certain data support for the application of CS-SeNPs in preservation.