Abstract: Through an orthogonal design L16(45), the optimized preparing condition of polysaccharide from Sargassum thunbergii was found at pH=4, applying ultrasound at 800W for 200s, incubating at 100℃ for 6h, and 3% (W/V) of trichloroacetic acid and 70% (V/V) of ethanol. The extraction yield of polysaccharide was up to 6.24%. The crude polysaccharide was purified with Sepharose 4B column chromatography and the antioxidant activity of polysaccharide from Sargassum thunbergii was determined by radical scavenging methods. The result showed that the polysaccharide had satisfactory antioxidant activity: the scavenging rate was more than 50% towards DPPH·, and was also more than 30% towards ·OH and O2-

Key words: Sargassum thunbergii, polysaccharide, preparation, antioxidant activity -