In this study, the phosphorylated derivative of low-molecular-weight polysaccharide from Enteromorpha linza was prepared. And then the antioxidant activities of all the samples were investigated including scavenging effects of superoxide and hydroxyl radicals and reducing power. The results of chemical composition, FT-IR spectrum and DSC analysis showed the modifications of polysaccharide were successful. And moreover, the phosphorylated derivative showed excellent antioxidant activity in these three assays, so this derivative needs to be attention in further.