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The longitudinal axial flow corn kernel harvester threshing separation device has high grain breakage rate during the threshing process, low removal rate, large loss of grain entrainment, and problem of clogging the concave plate by corn leaf. Aiming at the above problems, based on the collision contact theory, a degranulation separation device with low-damage round head nail teeth and segmented combined round tube threshing concave plate was designed. Combined with the physical properties of corn kernels, the relative critical velocity of the mechanical stress damage of the corn, the structural parameters of the optimal threshing element and the optimal gravure combination were determined. Based on the test factors of grain breakage rate and free removal rate, a comparison test was carried out with a conventional threshing rod and grid concave plate. It was determined through experiments that the optimal threshing element was a round head nail (equal height) and the optimal ball head radius was 12.5mm. The best gravure combination form was the right pipe and straight pipe (front sparse and rear dense). Under the optimal combination of parameters, the grain breakage rate of the threshing device was decreased from 13.73% to 8.64%;the unremoved rate was decreased from 0.6% to 0.2%. The device offered an effective solution for the problem of conventional grid gravure blocked by corn stalks, which met the design requirements and provided theoretical guidance for the design of corn threshing separation.