Abstract: In this paper,authors assess vitro antioxidation by studying cleaning effect of·O₂^- and·OH and compare trichloroacetic acid (TCA) method with Sevage method by analyzing deproteinization rate and polysaccharides loss rate. Then the authors purify polysaccharides and determine the distribution of molecular weight through using glucan gel G-150 chromatography column,furthermore analyze some of its physical and chemical properties. The results indicate that polysaccharides from Grateloupia filicin has antioxidant activity,IC₅₀ on scavenging·OH is 4.62 mg/mL,IC₅₀ on scavenging·O₂^- is 0.62 mg/mL. On effect of deproteinization,Sevage method is superior to TCA method,deproteinization rate is 85.55% and polysaccharides loss rate is 15.93%. Polysaccharides from Grateloupia filicin is a kind of compounds with major constituent polysaccharides and glycoprotein,and the distribution of molecular weight is between 3.5×10⁵ and 1.2×10⁵ Da.