AbstractPurple sweet potato polysaccharide was extracted via hot water, and it was chemically modified by phosphorus oxychloride‐pyridine to obtain phosphorylated polysaccharide from purple sweet potato (P‐PPSP) with certain degrees of substitution. Furthermore, the structure and antioxidant activity in vitro of PPSP and phosphorylated derivative were compared. The result indicated that the phosphorylation modification product of polysaccharide from purple sweet potato could improve the scavenging effect on hydroxyl radical and superoxide anion of PPSP, significantly. It also could improve the anti‐lipid peroxidation ability while fail to improve the reducing ability of PPSP.