To discuss the effect of polysaccharide molecular weight on the antioxidation, Gracilaria lemaneiformis polysaccharide was degraded by free radical from ascorbate and hydrogen peroxide in combination, then the antioxidative activity of G. lemaneiformis polysaccharide and its degraded products was determined. The results showed that G. lemaneiformis polysaccharide was degraded basically by the reaction system of ascorbate and hydrogen peroxide in two hours, and four kinds of degraded polysaccharide were achieved with molecular size of 21 492，12 864，5 417 and 3 043 u, respectively. In the degradation, the sulfate contents were not changed but 3,6anhydrogalactose contents of degraded products decreased a little. Moreover, the higher the molecular weight of G. lemaneiformis polysaccharide, the weaker the power of reducing Fe³⁺, scavenging superoxide anion radical (O₂^-·) and 1,1-diphenyl2-picrylhydrazyl (DPPH) radical. However, the inhibitory activity againest hydroxyl radical (·OH) decreased with the degradation.