Abstract: It is very important for sweet potatoes used for extracting starch to be harvested and processed in time. The paper put forward a method to determine the optimum period for harvesting sweet potatoes, and the optimum harvest periods of two kinds of sweet potatoes were determined. In order to process sweet potatoes during their optimum harvest period and to increase the sweet potato starch, a small scale 6SF-20 potato grinding starch extracting machine was developed with starch separating rate of 96.4% and starch extracting rate of 92.0%. All the indices come up to the world advanced standards.