Abstract: As large harvesters cannot easily pass through in southern hilly area, and the threshing part of horizontal axial flow harvesters causes big loss rate, we designed a small vertical axial flow threshing device of rice harvester. It was assembled to the harvester to test the loss rate of threshing part in field. The results showed that tooth pitch of elastic threshing pole, helix angle of conducting bar and dip angle of vertical bar on concave plate screen had significant effect on the loss rate of rice. The optimal parameter combination of the system was tooth pitch of elastic threshing pole was 90 mm, helix angle of conducting bar was 23°, dip angle of vertical bar on concave plate screen was 14°. Under the above conditions, the loss rate of rice was 1.51%, which was less than the national standard (3%). The test data provides a technical support for industrialization of small vertical axis flow rice combine harvester.