Abstract: Objective: To optimize the preparation process of dextran nano-selenium (DEX-SeNPs), detect its morphological characteristics, and evaluate its antioxidant capacity in vitro, while exploring the mechanism of dextran (DEX) stabilizing nano-selenium (SeNPs). Methods: DEX-SeNPs were prepared using DXE as a dispersant and stabilizer through a chemical reduction method. The preparation process was investigated using single-factor tests. The structure was characterized using particle size analyzers, UV spectroscopy, IR spectroscopy, and X-ray photoelectron spectroscopy. The stability mechanism was further discussed, and the hydroxyl radical scavenging ability was explored. Results: The optimal conditions for preparing DEX-SeNPs were a molar ratio of ascorbic acid to sodium selenite of 4:1, a reaction temperature of 40 ℃, and a reaction time of 1 hour. DEX-SeNPs were identified as amorphous zero-valent selenium, uniformly spherical in shape, with an average size of 114 nm and a polydispersity index of 0.038. They maintained a stable colloidal solution state at 4 ℃ for 30 days and exhibited good hydroxyl radical scavenging ability. The possible mechanism for DEX stabilizing SeNPs was the combination of selenium atoms on the surface of SeNPs with hydroxyl groups of DEX, forming Se···HO hydrogen bonds to prevent aggregation of SeNPs. Conclusion: The preparation of DEX-SeNPs provided theoretical data support for the development of polysaccharide-based SeNPs selenium supplements and research on their biological activity applications.