In this study, a novel water-soluble exopolysaccharide—dextran, biosynthesized by Leuconostoc Mesenteroides CICC-21725 was separated, purified, and comprehensively characterized. The yield of purified dextran was 30.50 g/L and, the weight-average molecular weight (Mw) was 8.39 × 107 Da. The structure of the purified dextran determined by FT-IR, and 1H,13C NMR spectroscopy demonstrated that the dextran was of α dextran type, mainly containing α-(1→6) glycosidic bonds. The morphological observation by SEM and AFM elucidated that dextran was a cluster of micro-nano size irregular spherical particles with a rough surface. The purified dextran obtained has good thermal stability. The water absorption rate of the dextran product was 22.61%, and the viscosity was 10.17 mPas.