Abstract: Auricularia polytricha polysaccharide( APP3a) was carboxymethylated by the method of alkaline chloroacetic acid and its carboxymethylation process was studied.The process parameters including chloroacetic acid amount,sodium hydroxide amount,reaction time,temperature,were studied through the single factor(experiment,and the optimization of carboxymethylation were adopted by response surface experiments with Box- Behnken design. The experimental results showed that the optimal conditions of Auricularia polytricha polysaccharide( APP3a) carboxymethylation were obtained as follows: isopropyl alcohol used as reaction medium,60 mg of APP3 a,1.45 g of chloroacetic acid,2.48 g of sodium hydroxide,55 ℃ of reaction temperature,3.5 h of reaction time. Under this conditions the maximum substitution degree of 0.892 was obtained. Compared to the theoretical value,the relative error was 0.33%. It showed that the regression equation by response surface optimization had certain practical significance.