Abstract: This paper studied that it was feasible to apply membrane separation technology to extract fucoidan from kelp polysaccharide.Also, the best technical parameters had been fixed on.It demonstrated that extraction in cold water could protect fucoidan from degradation.More than 80% of alginate could be removed at the condition of pH≤2.5.With the increasing of operating pressure, membrane flux would increase during the ultrafiltration process.After diluting the extract 0～2times, membrane flux increased, but the extracting ratio almost didn't changed.The results showed that the best extracting condition were:temperature 4℃, rate of material to liquid 1:30, extract 5 hours.Alginate removed under the condition of pH=2.0 50ku of ultrafiltration membrane was used to purify fucoidan under the pressure of 0.2MPa.The permeated liquid was precipitated by ethanol and dried by vacuum-freeze drying.The yield of fucoidan was 2.32%.The compositions of total sugar and sulphate were 38.24% and 21.49%.