Three polysaccharide fractions from fruiting bodies of Coprinus comatus-CC30, CC60, and CC80-are obtained by water extraction and ethanol precipitation with ethanol percentages of 30%, 60%, and 80%, respectively. The hypoglycemic activity of the three fractions was tested based on mice with alloxan-induced diabetes. Results indicate that fraction CC60 is the most effective fraction in water extract from C. comatus; it can remarkably reduce the blood glucose concentration in 120 min at a dosage of 1000 mg/kg administered orally. It also presents a long-term hypoglycemic effect during 21 days of injection at the same dosage. This polysaccharide fraction provide a novel path to improve the treatment currently used for patients with diabetes. The data on mice spleen lymphocyte proliferation and protein tyrosine phosphatase 1B (PTP1B)-inhibiting activity of fractions indicate that the hypoglycemic activity of CC60 is possibly activated through immune stimulation, not PTP1B inhibition.