Using rice starch and sodium chloroacetate as raw material, carboxymethyl starch (CMS) was obtained by semi-dry method. The influences of dosage of sodium chloroacetate and NaOH, reaction temperature and time of etherification, alkalinization temperature conditions on the substitution degree of product were investigated, and then the infrared spectrum and crystal style of the CMS were analyzed. The results showed that optimizing conditions of CMS by single factor experiment were starch and sodium chloroacetate of 11 in molar ratio, starch and sodium hydroxide of 11.25 in molar ratio, alkalinized at 35 ℃ for 1 h, and then etherified at 60 ℃ for 2.5 h. Finally, the substitution degree of obtained CMS was 0.45 under the above conditions, and a new absorption peak, the characterisitic one of carboxyl functional groups, appeared between 1 300 and 1 600 cm^-1. The crystal structure of the starch changed from polycrystalline granules to be amorphous.