Abstract: The optimal reaction conditions of preparing carboxylmethyl alkali soluble polysaccharides from Dioscorea fordii Prain et Burkill (DFPA-Ⅰ) and vitriol esterified DFPA-Ⅰ were determined.The substitution degree of carboxylmethyl DFPA-Ⅰ was 0.55.The water solubility of carboxylmethyl DFPA-Ⅰ was 3.4 g, or 17 times of that of DFPA-Ⅰ.The study showed that the adsorption for Hg2+of carboxylmethyl DFPA-Ⅰ was 4.6 times of that of DFPA-Ⅰ. The substitution degree of vitriol esterified DFPA-Ⅰ was 1.06.Water solubility of vitriol esterfied DFPA-Ⅰ was 4.1 g, or 20.5 times of that of DFPA-Ⅰ. Effect in scavenging superoxide radicals generated from self-oxidation of pyrogallic acid by vitriol esterified DFPA-Ⅰ was 4 times of that of DFPA-Ⅰ.Carboxylmethyl DFPA-Ⅰ and vitriol esterified DFPA-Ⅰ have hypoglycemic activities on alloxan-induced diabetic mice,and their hypoglycemic activities are higher than that of DFPA-Ⅰ. With dosages of 0.03 and 0.1 g/kg by DFPA-Ⅰ, carboxyl-methyl DFPA-Ⅰ and vitriol esterified DFPA-Ⅰ respectively for 7 days, blood sugar in diabetic mice can be decreased by 33.7% and 45.2%, 54.6%and 63.8%, 68.2% and 89.4% respectively.