Polysaccharides were extracted from Porphyra yezoensis using ultrasonic extraction method and were purified the polysaccharides with DEAE-52 and Sephadex G-200 chromatogram. Through orthogonal tests,we achieved the best conditions of ultrasonic extraction process as follows: weight of Porphyra yezoensis is 15 g, volume of water is 2 L, rotation speed of pump is 12 r/s, extraction temperature is 40℃, ultrasonic power is 800W, time ratio of ultrasonic processing and intermission is 3s/2s and ultrasonic processing time is 2 h. In this method, production rate of rough polysaccharides reaches 26.29%. Then we precipitated rough polysaccharides in order with 50%, 60%, 70% and 80%(V/V) ethanol and got four ingredients of rough polysaccharides. These ingredients were purified with DEAE-52 and Sephadex G-200 chromatogram and then 5 or 3 kinds of different molecular weight polysaccharides were obtained, respectively. The former parts have a little inhibition effects on U937 cell growth, yet the latter parts have promotion effects.