Abstract: This paper mainly used corn starch as raw material,ethanol as solvent,chloroacetic acid as etherification agent,to study the influence of various factors on the degree of substitution(DS) respectively. The processing conditions of preparing carboxymethyl starch(CMS) with high degree of substitution was as follows :sodium hydroxide was added for two times,95%(mass fraction) ethanol as solvent,starch slurry concentration was25 %, 115 g chloroacetic acid, dosage of sodium hydroxide was 2. 25( molar ratio, alkali : acid), 40 ℃ was adopted to alkalify starch,alkalization for 10 h,sodium hydroxide for alkalization was 1(molar ratio,alkali∶acid),40 ℃ was used to etherify starch,kept the etherification for 10 h,14 g sodium carbonate was used to replace part of sodium hydroxide during etherification phase. In addition,comparison of effects on DS of different types of starches,etherifying agents and sodium hydroxide states were also had been discussed. Results showed that the carboxymethyl starch,which DS was 1.21 and reaction efficiency(RE) was 61.38%,was attained by one step method. DS of non-crystal granular starch was slightly improved compared with native starch.According to DS from high to low,it was potato starch,cassava starch,waxy corn starch,corn starch successively that were used to prepare CMS. DS of CMS based on chloroacetic acid as etherification agent was much higher than sodium chloroacetate,and DS of CMS prepared by solid alkali increased dramatically compared with liquid alkali.