Abstract: In this paper, Undaria pinnatifida polysaccharides ( UPPS) were extracted by ultrasonic-assisted method. The content of polysaccharides was measured by phenol-sulfate method. Effects of three factors on the extraction, including extraction temperature, extraction time and solid/solvent ratio were studied by single factor and the optimum extraction conditions were then determined through the orthogonal experiment.Meanwhile, physical and chemical properties of the resultant polysaccharide were also concerned in this paper.Results showed that these optimum extraction conditions as follows :solid/liquid ratio 1∶45 (g/mL) , extraction temperature 80℃, and extracting duration 40min. Under such conditions, the yield of the polysaccharide reached 7.56%. Physicochemical analysis revealed that the polysaccharide contained protein and sulfate radicals, but no starch was detected in it. The structure of the Undaria pinnatifida polysaccharides remained intact after exposure to high temperature or H3BO4. However, the polysaccharide was slightly affected by acid or alkali and the conformation of the polysaccharide was significantly damaged by Ca2+.