The ultrasonic wave was used in the pretreatment of corn starch. The ultrasonic effect on the corn starch molecular weight distribution was investigated. The ultrasound technology was then combined with the traditional wet process. The result indicated that the ultrasound treatment decreased molecular weight of amylopectin. The optimum conditions of ultrasonic pretreatment method obtained by response surface design were as follows: reaction temperature 42. 58 ℃,pH 12. 03,ultrasound treatment time 47. 11 min,ultrasonic power 150 W. Under the above conditions,the actual substitution degree was 0. 0512,the reaction efficiency was 74. 25%.