Poria cocos (Polyporacea), is a fungus used in traditional Chinese medicine. A study of the valuable sulfated polysaccharides (SPS) with the structure and pharmaceutical benefits from the mycelial culture conditions of P. cocos was attempted. The SPSs were fractionated by gel filtration chromatography to give a fucose-containing mannoglucan polysaccharide (denoted as FMGP): The main skeleton was a 1,4-α-Man-interlaced-1,3-β-glucan with interlaced 6-O-α-l-fucosyl 1,4-α-Glc and 1,4-α-Gal branches. FMGP dramatically inhibited cell migration in the highly metastatic human lung cancer cell line CL1-5 cells. Mechanistically, FMGP dramatically downregulated the expression of TGFβRI and inhibited phosphorylation of FAK and AKT. Moreover, FMGP reduced the metastasis-related protein, Slug, expression. This is the first paper reporting a branched 1,3-β-mannoglucan from P. cocos and its anti-lung cancer CL1-5 cells migration activities.