Carboxymethyl inulin(CMI) was prepared by ultrasonic-assisted method in NaOH-chloroacetic acid with isopropanol as the solvent.The effects of etherification time,alkalization time,solvent volume fraction and amount of NaOH on the degree of substitution(DS) of CMI were investigated.The suitable conditions were as follows:m(inulin):m(chloroacetic acid):m(sodium hydroxide) =2.75:1.4:1,alkalization time 4 minutes,etherification time 15 minutes.The DS of CMI was obtained as 0.41 under the optimum reaction condition.Structure confirmation of CMI was identified by infrared spectroscopy(IR) and X-ray diffracttometry(X-RD),which have proved that the modification method is feasible.