The invention relates to a method for preparing porphyra polysaccharide and use thereof. The method comprises steps of crushing a porphyra, adding water to boil and simmer the porphyra for a period of time to form a mixture repeating the boil and simmer steps at least twice centrifuging the mixture to separating a filtrate and an insoluble matter adding a low-carbon alcohol to the filtrate for precipitating polysaccharide to form a precipitate and finally freeze-drying the precipitate to manufacture a porphyra polysaccharide having ability to scavenge free radicals and retain moisture. Accordingly, the porphyra polysaccharide can further be used as a cosmetic composition for antioxidant or moisturizing.本發明係有關於一種紫醣素(porphyra polysaccharide)之製備方法及其用途,係先將紫菜(porphyra)打成泥狀物,加水煮沸並悶煮一段時間以形成一混合液;再重複煮沸及悶煮步驟至少兩次;離心混合液以分離出一濾液以及一不溶物;取濾液加入低碳醇以沉澱多醣,形成一沉澱物,最後冷凍乾燥沉澱物以製得具有清除自由基以及保濕能力的紫醣素;藉此,此紫醣素可進一步作為抗氧化或保濕之化妝材料組成物。(S1)‧‧‧步驟一(S2)‧‧‧步驟二(S3)‧‧‧步驟三(S4)‧‧‧步驟四(S5)‧‧‧步驟五
