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Aiming to probe new principle and parts of soybean seed thresher and solve the conflict between high seed damage and no-threshed rate during threshing, a new soybean seed thresher with tangential-axial flow double-roller was designed based on the new threshing principle and structure. From the overall structure, the threshing device was designed, which was composed of a couple of primary and secondary threshing rollers with different diameters, lengths, concave screen and transmission system. The double rollers of the thresher were configured by tangential-axial flow form. Compared with the traditional threshing ways with single threshing, the new soybean thresher was combined several threshing principles and two rollers with two kinds of threshing parts, the secondary threshing roller consisted of spike teeth was used as pre-thresh of soybean plants, and the primary threshing roller consisted of bow-spike teeth, which threshed and separated most of the plants in the axial threshing space. The double-roller operated with different linear velocities which realized the ordered feeding and fast grabbing of soybean plants. In order to check and find the optimal structure and parameters of the soybean seed thresher, performance test of the thresher was conducted under the condition that the moisture content of soybean seed was 17%~19%, the moisture content of soybean straw was 12%~15% and the straw-grain ratio was 1.275. Three parameters, including feeding quantity, rotating speed of double-roller and concave clearance were selected as input variables and the damage rate and un-threshed rate as output parameter. Meanwhile, a drastic orthogonal rotary regressive experimental design was employed to develop the second order polynomial regression models, which explained the relationship between the input and output parameters, and then the main parameters of the thresher was optimized through Design-Expert 8.0 software. Experimental results showed that feeding quantify, rotating speed of double-roller and concave clearance had an effect on the damage rate and un-threshed rate, and the rotating speed of double-roller was the most important influence element among the three. Optimization analysis indicated that the most optimum combination was the feeding quantity of 0.44kg/s, the rotating speed of double-roller of 489r/min and the concave clearance of 25.06mm, and the damage rate was 1.18% and the un-threshed rate was 0.65%. Compared with the traditional soybean threshing machine, it can make the damage rate and un-threshed rate decreased by 0.22 percentage points and 0.38 percentage points, respectively.