Polysaccharides,named MY,YP1,YP2 respectively,was isolated from fresh Chinese yam with water(70℃,Solid-liquid ratio=1∶6),alcohol sedimentation(80%,v/v),DEAE-52 column.The inhibition of their activity on α-amylase was studied.The results show that MY inhibits α-amylase activity to a certain extent.MY was purified and identified by Sephacryl S-400 named as MYP.The carbohydrate moiety contained arabinose,glucose,galactose at a mass ratio of 1∶20.7∶3.3.It was also rich in protein,such as leu,glu,arg,asp.With the method of NMR,we found the probable chemical construction of MYP was:glucose link with glycosidic bond atβ-1→2 and β-1→4,galactose and arabinose was linked at the gap or lateral chain by β-1→2 or α-1→2.