A water soluble polysaccharide (HPⅠ) was isolated from the Hericium caput-medusae and examined homogenously by Sephadex G-100 column chromatography,cellulose acetate film electrophoresis and specific optical rotation measurement. This HPⅠconsists of Fuc,Glc and Gal in the molar ratios of 0.423∶1∶2.110. Its structural features were investigated by methylation and GC-MS analysis,periodate oxidation-Smith degradation,and partial acid-hydrolysis. In addition, enzymatic degradation experiments and GC analysis were applied. The following structure of the highly branched repeating unit was established: 6→)-Gal-(1→6)-Gal-(1→6)-Gal-(1→6)-Gal-(1→6)-Gal-(1→6)-Gal-(1→6)-Gal-(1→6)-Gal-(1→6)-Gal-(1→ 2 ↑ 1 2 ↑ 1 2 ↑ 1 Gal-(1→4)-Fuc Glc-(1→3)-Glc 2 Gal-(1→6)-Glc 3 The activities of HPⅠwere tested in mice. All the samples exhibited high effect of prevention and cure against chronic gastricism.