The invention discloses a gastrodia elata BL. polysaccharide WGE extracted from a gastrodia elata BL. bulk pharmaceutical, degradation products WGE-AD-I and WGE-AD-O of the gastrodia elata BL. polysaccharide WGE, preparation methods of the degradation products and applications of the degradation products to preparation of antitumor medicaments. The structural formula of the gastrodia elata BL. polysaccharide is shown in the specifications, wherein x and y are integers, and x+y=16; n is an integer; an average molecular weight is 1,000 kD (the average molecular weight range is 800-1,200 kD); and the specific rotation is [alpha] 19D+382 degrees (c0.05, H2O). The structural repeat units of degradation product polysaccharides WGE-AD-I obtained by performing partial acid hydrolysis on the gastrodia elata BL. polysaccharide are the same as those of the WGE, but the average molecular weight of the degradation product polysaccharides WGE-AD-I is 10 kD (the average molecular weight range is 8-12 kD). Another degradation product WGE-AD-O is a mixture of olig-dextran (the molecular weight range is 320-3,000 Dalton). As proved by in-vitro experiments, the gastrodia elata BL. polysaccharide and the degradation products thereof can be used for remarkably restraining the growth of pancreatic cancer cells and melanoma cells, have hardly any toxicity on normal cells, and are hopeful to be developed into antitumor medicaments.