To optimize the sulfated modification conditions of Undaria pinnatifida polysaccharides (UPPSⅠB). UPPSⅠB was modified by chlorosulfonic acid-pyridine method. L9(34) orthogonal test was used to optimize the reaction temperature, reagent ratio and reaction time with the degree of sulfate group and carbohydrate content as index. The major factor was reaction temperature. The optimal modification conditions were that the reaction temperature was 60°C, the ratio of chlorosulfonic acid to pyridine was 1:2, and the reaction time was 1.5h. The sulfated UPPSⅠB could be obtained using the optimal modification conditions.