Abstract: Objective: To explore the stabilization and modification mechanisms of Polysachcaride of Nostoc sphaeroides Küting (PNSK) on nano-selenium particles (SeNP), and the changes in biological activity before and after modification. Method: Using polysaccharide of PNSK as the raw material, polysachcaride of Nostoc sphaeroides Küting nano-selenium particles (PNSK-SeNP) was prepared by the sodium delenite (Na₂SeO₃) and ascorbic acid (V_C) redox method. The structure of PNSK and PNSK-SeNP were characterized by UV-Vis spectroscopy, high-performance liquid chromatography, X-ray diffraction, Infrared spectroscopy, and X-ray photoelectron spectroscopy. The antioxidant and anticancer activity of PNSK and PNSK-SeNP were comparatively analyzed. Results: The results showed that the selenium content of PNSK-SeNP was 52.03±0.20 mg/g and the particle size was 166.83±4.51 nm. No obvious difference was observed in the molar ratio of monosaccharides of PNSK-SeNP before and after preparation. The presence of a Se 3d signal peak in the X-ray photoelectron spectroscopy of PNSK-SeNP, and the selenium element existed in the 0-valent state. Both infrared spectroscopy and X-ray photoelectron spectroscopy indicated that the main structure and functional groups of PNSK were preserved in PNSK-SeNP, and no new chemical bonds were formed between SeNP and PNSK. The antioxidant assay found that the IC₅₀ of PNSK and PNSK-SeNP for hydroxyl radical (·OH) were 4.45 and 0.55 mg/mL, respectively, and for 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid), diammonium salt radicals (ABTS) were 4.58 and 0.79 mg/mL, respectively, and for the A549 lung cancer cells were 0.87 and 0.06 mg/mL, respectively. Meanwhile, the migration of A549 lung cancer cells can be effectively inhibited by PNSK-SeNP, and apoptosis can also be induced. A significant correlation between the selenium content of PNSK-SeNP and its antioxidant and anticancer effects were revealed by correlation analysis. Conclusion: Nano-selenium can be effectively stabilized by PNSK, and the in vitro antioxidant and anti-cancer activities of PNSK-SeNP can be significantly enhanced compared with PNSK.