Abstract: The preparation and dynamic property of sodium carboxymethyl starch (CMS-Na) were studied by using with sweet potato starch as raw material, sodium chloroacetate as etherization reagent, sodium hydroxide as catalyst and aqueous solutions of ethanol as solvent. The effects of some factors on preparation, such as dosage of sodium hydroxide and sodium chloroacetate, reaction temperature, reaction time on the substitution degree of the sodium carboxymethyl starch were investigated. The results indicated that CMS-Na was preferably prepared as follows:sweet potato starch 50g, sodium chloroacetate 15g and sodium hydroxide 7.8g at 49℃ for 2.5h. The kinetic study showed that the etherification between sweet potato starch and sodium chloroacetate accorded with the first order reaction kinetic model.