In order to discuss threshing mechanism of axial threshing space further from the view of mechanics, the authors explored a dynamical model of axial flow threshing device with plunk-tooth and helical vanes assembled. Based on the principle of dynamics, mechanical condition of threshing grain was analyzed, and the force that loaded on the grain was simulated in the course of threshing. Results show that the force is uneven along the edge of helical vanes. When grain moves to the area with the range from five to nine meters near to feeding entrance along helical vanes, the acceleration reaches the maximum areas during the threshing period. The force on the grain between the concave board, cover board and cylinder is evidently higher than that on the grain in other space, which indicates that the parameaters of cover board, concave board are main factors of axcial flow threshing.