Cortex mori radices, the dried root cortex of Morus alba L., is a clinically widely used traditional Chinese medicine, with the effects of anti-tussive, preventing asthma, hypoglycemic, antitumor, antibacterial and antioxidative. Up to now, its polysaccharides and their contribution to biological activities remain largely unknown. In this study, the root cortex was extracted with boiling water, followed by ethanol precipitation, fractionation and purification by anion-exchange chromatography on DEAE-cellulose column and gel permeation chromatography on Sephacryl S-300 column. Three polysaccharides, CMA-a-1, CMA-a-5 and CMA-b1-1, were isolated from boiling-water extract, and characterized to be homogeneous fractions by high performance gel permeation chromatography (HPGPC). Their structural features were elucidated by sugar analyses, methylation analyses, IR, and NMR spectroscopic methods. Both CMA-a-1 and CMA-a-5 are found to be starch fractions. CMA-a-1, with an average molecular weight (MW) of 224 kDa, is an amylopectin-like polysaccharide with an average chain length (CL) of 23, containing 1,4-linked α-D-glucopyranosyl backbone, with 1, 6-linked glucose in the side chains attached to O-6 of backbone residues. CMA-a-5, with a MW of 15.5 kDa, is mainly an amylose fraction with an average chain length of 30. CMA-b1-1 was isolated from 0.2 mol·L^-1 NaCl-eluted fraction upon anion-exchange chromatography, and characterized as a RG-I type pectic polysaccharide, which contains a backbone composed of alternating 1,4-linked α-D-galacturonic acid and 1,2-linked α-L-rhamnose residues, with various branches attached to O-4 of rhamnose residues, comprising of T-, 1,3-, 1,5-, 1,3,5-linked Araf, T-,1,6-, 1,3,6-linked Galp, 1, 4-linked Xylp, and 1, 4-linked Glcp. MTT method showed that CMA-b1-1 could inhibit the proliferation of SMMC7721 liver cancer cells and showed no impairment to L02 normal liver cells. This study suggested that the boiling water extract of cortex mori radices contains mainly starch and pectic fraction as the major component polysaccharides, and that the RG-I polysaccharide in vitro can inhibit the liver cancer cell proliferation, but the starch fractions cannot.