Abstract: Objective: The antioxidant activities and main structural characteristics of water-insoluble and water-soluble polysaccharides of Euglena gracilis were comparatively studied, which provided an experimental basis for the subsequent research of the structure-activity relationship of E. gracilis polysaccharides. Methods: The water-insoluble and water-soluble polysaccharides were extracted, separated and purified from E. gracilis. The basic structure of the polysaccharides was analyzed by UV, IR, HPGPC and IC techniques. The antioxidant activities of these polysaccharides were further investigated by measuring scavenging activities against 1,1-Diphenyl-2-picrylhydrazyl free radical (DPPH·) and hydroxyl radical (·OH), and reducing ability, compared with vitamin C (V_C). Results: The molecular weights of E. gracilis water-insoluble polysaccharides (EGP-1) and E. gracilis water-soluble polysaccharides (EGP-2A-1 and EGP-2B-1) were 1.01×10⁴, 2.91×10⁶ and 1.70×10⁶ Da, respectively. EGP-1 was a glucopyranose cyclic polysaccharide connected by β-glycosidic bonds, and EGP-2A-1 and EGP-2B-1 were glucopyranose cyclic polysaccharides containing α- and β-glycosidic bonds. The main monosaccharides found in EGP-1 were glucose, mannose, fucose and rhamnose; EGP-2A-1 and EGP-2B-1 were composed of fucose, rhamnose, arabinose, glucosamine, hydrochloride galactose, glucose, xylose, mannose and glucuronic acid. The results of antioxidant activities showed that three-component polysaccharides had antioxidant activity, the order of their antioxidant capacities was V_C>EGP-2A-1>EGP-2B-1>EGP-1. Conclusions: Both the water-insoluble and water-soluble polysaccharides isolated from E. gracilis had antioxidant capacity, and the antioxidant activity of water-soluble polysaccharides was stronger than that of water-insoluble polysaccharides, and the EGP-2A-1 was the best.