In this article, some factors, such as ultrasound time, ultrasound power and starch concentration, affecting corn starch paste’s viscosity and morphology of starch particle are studied by use of ultrasound. The starch treated by ultrasound is used to prepare corn hydroxypropyl starch (HPS) with propylene oxide under microwave. Compared with conventional methods, ultrasonic-microwave method can improve HPS’s degree of molar substitution (MS) and sharply shorten reaction time. It is found that ultrasound can destruct surface morphology of starch particle, which can increase reaction efficiency as the particle becomes relax and easily to react with propylene oxide. HPS’s MS increases along with the reaction time and concentration of propylene oxide. With the increment of starch concentration and microwave power, the HPS’s MS increases at beginning and then decreases. The optimal parameters for HPS by use of ultrasonic-microwave irradiation are as followings: ultrasound power is 600 W, ultrasound-treated time is 25 min, microwave power is 100 W, microwave-treated time is 14 min, starch concentration is 35% and concentration of propylene oxide is 6.4%.