Abstract:The aim of the study is to explore the antioxidant activity of Porphyra Haitanensis polysaccharide with different molecular weight in vivo，and therefore provide theoretical basis for the optimizations and application of development techniques of low-value Porphyra Haitanensis.Through ultrafiltration，five polysaccharides with different molecular masses were extracted form four ultrafiltration membranes with different molecular weight cut，PPⅡ and PPⅣ of them were used in antioxidant activity study as their had strong in vivo activity.The results showed that：after 60 days when the mice were injected with D-galactose continuously，the activities of SOD，GSH-Px and CAT in the liver tissue，brain tissue and blood of the mice in the aging model group all decreased significantly.MDA content，on the other had，significantly increased.Which indicated the aging mice model was suited to the study.Moderate and high dose groups of PPⅡ and PPⅣ could significantly improve the SOD，GSH-Px and CAT activities in the liver tissue，brain tissue and blood to varying degrees.Similar doses also reduced the MDA content.Taken together these dates suggest a preferable antioxidant effect.However，most of the antioxidant indexes of PPⅣ group were obviously better than those of PPⅡ group.Above results will benefit to the deep-processing of Porphyra Haitanensis.