Abstract: A strain of yeast A9 was screened out from koumiss in Xinlingol, Inner Mongolia.The best conditions for mutagen of A9 were investigated by UV and single-factor test of DES. The results showed that, by UV screening experiment, the optimal parameters of UV was confirmed.Under the 30 W UV lamp, distance from UV lamp 30 cm, shining 75 s, the fatality rate of UV mutation was 78.9%.The optimal parameters were DES of 1% and shocked conditions of water-bath shocking box at the 35 ℃for 15 min, and fatality rate of DES mutation was 76.1%.After three times continuous UV-DES compound mutations, 4 mutant stains were selected to screen of high sugar-tolerant with A9. When the glucose concentration was 65% in the high-sugar-tolerant medium, the growth of A9 was suppressed but the A9-2 still grown under the same conditions.The high-sugar-tolerant performance of A9-2 was better than A9, so the best high-sugar-tolerant strain was A9-2.By the experiment of alcoholic fermentation, the weight loss of CO2, residual sugar, and alcoholic strength of the tenth fermentation of A9-2 were 13.20 g, 13.9 °Bé and 5.1% ( v/v) , which was similar to the third fermentation and its heredity performance was stable.The fermentation performance of A9-2 was higher than that of the original strain A9 obviously.