Abstract: Based on the single factor experiments, response surface methodology was employed to design, optimize and verify the culture medium for a endophytic fungus producing escin which was numbered as Irpex sp. EA-LJS-73, so as to improve the yield of escin. The results of optimum medium were as follows:starch 22.660 g, yeast extract 4.230 g, and MgSO₄ 7.600 g. Under these conditions, the predicted values of escin concentration could reach 1.116 g/L, the average concentration of the verification test was 1.105 g/L, which was 35% higher than before. Through the analysis of fermentation kinetics, when the strain fermented 10 d, both the biomass and the concentration of escin were the highest, which were 11.376 g (dry weight of the mycelium) and 1.665 g/L, respectively. Therefore, the optimized medium was reasonable and feasible.