Abstract: Carboxymethyl inulin (CMI) was prepared in the reaction system of NaOH-chloroacetic acid with isopropanol as the solvent.The antioxidative activity of CMI was studied.The preparation technology of CMI was optimized by single factor and orthogonal experiments with the evaluation index of substitution degree.The scavenging activities against hydroxyl free radicals and infrared spectrum of CMI were studied.The results showed that the suitable conditions were as follows:alkalization time 3min, chloroacetic acid amount 6.75g, isopropanol amount 4mL, etherification time 15min.The maximum substitution degree of CMI was 1.3327 under the optimum reaction condition.Meanwhile, CMI had strong scavenging capacities against hydroxyl free radicals with a dose-dependent effect.Structure confirmation by infrared spectroscopy (IR) proved that the preparation method of CMI was feasible.