To study the antitumor activity in vivo of Spirulina polysaccharide. Spirulina polysaccharide was extracted with double enzyme hydrolysis. H22 tumor bearing mice were established entity and treated by gavage with 1 000 mg/kg/d of the polysaccharide. The tumor weight, tumor inhibition rate, thymus index and Spleen index, lymphocyte proliferation, kill rate of NK cells, detection of phagocytic, the concentrations of TNF-αand IFN-γin mice blood serum were measured to study antitumor function in vivo. The tumor inhibition rate was 74.53%, The thymus index and Spleen index, lymphocyte proliferation, kill rate of NK cells were raised, and the concentrations of TNF-αand IFN-γ in mice blood serum were increased. The Spirulina polysaccharide had played an important role in protecting the immune system and promoting the immune function of tumor bearing mice.