Abstract: OBJECTIVE To extract pachyman polysaccharides(PPS) from Poria cocos, and determine the content of total polysaccharide in PPS. To obtain the carboxymethylpachymaran(CMP) by carboxymethylation modification of PPS, and evaluate the antitumor activity of PPS and CMP. METHODS Soaking extraction by diluted alkali solution was used to prepare PPS, and phenol-sulfuric method was used to measure the total polysaccharide. Preparation of CMP by carboxymethylation reaction. MTT assay was used to determine the cytotoxicity of PPS and CMP for HepG-2. RESULTS The content of the total polysaccharide in PPS was 95.96%, which determined by phenol-sulfuric method. Both of PPS and CMP inhibited the proliferation of HepG-2 cells, CMP had stronger activity. CONCLUSION The anti-tumor activity of CMP, which was modified with carboxymethyl, is more prominent than PPS.