The grain impurity rate of the combine harvester with across axial flow roller was high for the grain surplus could enter into the bin. By measuring the quantity and composition of grain surplus in unit time, the developed re-threshing system could calculate the speed and total amount of the material, and be tested in field. The statistics showed that this system could make the grain surplus amounting to 9%～10% smoothly be collected, transmitted, threshed, then sent into the bin again, which reduce the trash content from 7% to below 2%.