Abstract: Sulfur trioxide-pyridine method was used to prepare sulfated polysaccharide from carrots. With polysaccharide degree of substitution as the index,we studied the influences of reaction temperature,reaction time,and material ratio on the sulfation. The result indicated when the reaction temperature was 70℃,reaction time was 6h,material ratio was 1: 5,the conditions was the best. The scavenging effects of sulfated polysaccharide on the hydroxyl free radicals was significantly improved compare ed with untreated ones.