Abstract: The polysaccharides extracted from prasiola yunnanica.by hot water are EPS (extracellular polymeric substances).They are deproteinized by Sevag method for several times and purified by DEAE-cellulose and Sephadex G-200 chromatography column in turn.The crude polysaccharides were proved to have a little starch and some SO₄^2- by iodine indicator and BaCl₂ solution respectively.Agarose gel electrophoresis shows the existence of sugar and protein which are inhomoaenous.It has 13.54% of protein essayed by Lowry method, and 78% of sugar by benzene-H₂SO₄ method. And the rest are ash and water. The components of the crude polysaccharides were identified by paper chromatography and TLCwhich were D-Mannose, D-Ribose and L-Rhamnose with the ratio of 4: 15: 1. D-Ribose in the algal polysaccharides studies is quite rare.