Abstract: Enteromorpha intestinalis polysaccharides were modified by sulfamic acid- N,N- dimethyl formamide method to prepare different substitution degree( DS).And the relationship between DS and its antioxidant activity was evaluated by determining the scavenging ability of DPPH radical,hydroxyl fadical,superoxide anion radical and reducing power.The results indicated that the DS was affected by sulfated temperature,sulfated time,dosage of DMF and dosage of sulfamic acid. The dosage of sulfamic acid was the most significant factor. The antioxidant activity of E.intestinalis polysaccharides was increased at first and then decreased with the increasing of DS,and E.intestinalis polysaccharides showed good antioxidant activity when the DS was in the range of 0.8 ~ 1.2. It suggested that moderate sulfated modification was the most effective method to improve the antioxidant activity of E.intestinalis polysaccharides.