Objective： Streptococcus thermophilus is a kind of beneficial bacteria with obvious therapeutic effect on human body. In this experiment， 442 strains of S. thermophilus isolated from traditional fermented dairy products as research objects to screen out a high-yield extracellular polysaccharide strain MGD1-4 and the extracellular polysaccharide was isolated， purified and analyzed structurally. Methods： A few single fractions were purified from the extracellular polysaccharide produced by the fermentation broth of the strain， using the method of anionic chromatography of DEAE-Cellulose 52 and Sepharose CL-6B. And the structure was analyzed by gel permeation chromatography （GPC）， high performance liquid chromatography （HPLC）， infrared spectroscopy （IR） and nuclear magnetic resonance （NMR）. Results： It found that EPS-1a was a neutral polysaccharide with a molecular mass of 1.572×105 u， EPS-3a was an acidic polysaccharide with a molecular mass of 3.825×105 u， and both of them were single polysaccharide. The results of monosaccharide composition indicated that EPS-1a was mainly composed of glucose， mannose and galactose. The molar ratio of three was 3.62∶1∶2.99 and accounting for more than 90%， but EPS-3a was mainly composed of mannose， galactose and arabinose， which accounts for 80.6% of total， its molar ratio was 1.19∶1∶1.08. Combined with the results of 1D NMR， it was showing that the sugar ring of EPS-1a and EPS-3a were all pyranose rings， containing α glycosidic bond. Conclusion： By analyzing the structure of extracellular polysaccharide produced by the strain， the physical and chemical properties were initially determined， which is of great significance for further study of the structure-activity relationship of extracellular polysaccharide.