Crude polysaccharide was prepared from Crassostrea rivularis by 30% (w/v) potassium hydroxide solution at 90°C for 120 min. Three fractions (OG1, OG2, and OG3) were purified by DEAE‐52 cellulose and Sepharose 2B gel column chromatography. The chemical structures were determined using gas chromatography (GC), high‐performance gel permeation chromatography (HPGPC), Fourier‐transform infrared (FT‐IR) spectroscopy, and 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. The results indicated that OG1 was composed of rhamnose and little mannose (8.71%), the ratio of Rha: Gal: Xyl: Fuc in OG3 were 14:5.5:3:1. And their average molecular weights (Mw) were about 1.66 × 106 and 2.33 × 106 Da, respectively. OG2 was composed only of glucose (98.23%), which means it was glycogen. OG2 was consisted mainly of →4)‐ α‐D‐Glc‐(1→, with the branch chain every 6.5 glucose residues on average, which is →4,6)‐α‐D‐Glc‐(1→ and trace amount of α‐D‐Glc‐(1→ branched units. The Mw was 2.27 × 106 Da. It provides the bases for the bioactivity research.