Objective To study effect and mechanism of fungus polysaccharide PSM-a of Polyporus sp.M05 on S180 bearing mice. Methods MTT method was used to detect the inhibiting role of PSM-a on S180 cells proliferation in vitro. S180 mice model was established,and was administered by gavage. Tumor volume was detected, and the ratio of tumor to mile weight and inhibiting tumor rate. The activity of NK and LAK cells on the target cells was analyzed by MTT colorimetric assay ;HE stain was used to detect the necrosis of tumor cells. Results PSM-a could inhibit S180 cells grouth in vitro. PSM-a could decrease the tumor weight and increase the ratio of tumor volume and mice weight; Tumor inhibiting rate reached 80% and above when treated with 250 μg/nml PSM-a. PSM-a could increase the activity of NK and LAK cells, and necrosis happened. Conclusion PSM-a could significantly inhibit the growth of S180 cells, and the mechanism bnay be related with the increased killing activity of immunne cells to tumor cells.