The appropriate degradation of starch can expand its application scope. Compared with conventional chemical and enzymatic degradation methods, physical methods like ultrasonic treatment are green, convenient, and easy to operate. In this research, gelatinized normal corn starch was taken as the object, the degradation process and trend of the amylose-amylopectin mixed system was investigated in an ultrasonic field. After 30 min of ultrasonic treatment, the molecular weight of normal corn starch decreased from 1.68×107 g/mol to 8.33×105 g/mol, showing a trend of fast degradation followed by slow degradation. The results revealed a "size-dependent" degradation process for the starch molecules, that is, larger molecules were more easily degraded. Therefore, the degradation rate of amylopectin molecules was higher than that of amylose during the initial phase of ultrasonic treatment, and their degradation tended to be synchronized when their sizes reached the same. Based on the "intermediate degradation" of starch by ultrasound and the tendency that large molecules are more easily degraded, it was found that the Fb3 chain of starch molecules was broken down first, and as the ultrasonic treatment time increased, the Fb2 chain broke down, leading to the increase in the proportion of Fa chain. In addition, the breakage of α-1,4 glycosidic bonds of amylose led to the increases of Fb2 and Fb3 components. The 1H NMR results showed that both the α-1,4-glycosidic bond and the α-1,6-glycosidic bond of starch molecules were broken, but the breakage of the α-1,6-glycosidic bond was more significant. In summary, the ultrasound-induced degradation of the gelatinized amyloseamylopectin system was selective. The application of ultrasound as a means of physical modification in the starch industry is of great significance.