Abstract: In this paper, starting with the jam problem existed in the 5TD-680 thresher which is specially used for threshing soybean seed, and applying the dynamical theory, the mechanism of seed jams in the downspouting of thresher was analyzed. The mechanical formula for the downspouting of thresher was derived by applying mechanical theory and methods. According to the mechanical formula, its countermeasure was proposed. It indicated that the importance of omni-design for thresher which should include rational downspouting, the formula and the way in this paper could supply a reference for the design and research of downspouting of the thresher or other related agricultural implements.