Abstract: In this paper, the extraction technology of fucoidan from Undaria pinnatifida was optimized, then crude fucoidan was purified and the monosaccharide composition and sulfate ester content of each fraction were studied. On the basis of the Box-Behnken design in the Response Surface Analysis (RSA) method, the optimal technological conditions to extract fucoidan from Undaria pinnatifida were obtained as follows: extraction temperature: 76℃, extraction time: 2.5 h, algae/water ratio: 1︰6.5. Extracting twice under this condition, the extraction rate of polysaccharide was 7.53%. Then crude fucoidan was fractionalized into five fractions by a DEAE-Sepharose Fast Flow column. The sulfate ester contents of each fraction were 8.3%, 19.2%, 30.1%, 38.4% and 33.0%, respectively. The monosaccharide compositions of each fraction were as follows: the first fraction: mannose and galactose; the second fraction: rhamnose, fucose, xylose, mannose, glucose and galactose; the third fraction: rhamnose, fucose and mannose; the fourth fraction: fucose and galactose; and the fifth fraction: fucose, mannose and galactose.