Abstract: High unthreshed rate and low kernel breakage are often required for the harvesting of the high-moisture corn (30%-35%) in the Huang-Huai-Hai region. In this study, a longitudinal axial-flow threshing device was proposed using the rigid-flexible coupled damage reduction. A synergistically threshing structure was designed to mitigate the rigid impact between conventional threshing components and corn kernels. The rigid-flexible coupled threshing elements were integrated with the bionic thumb ones. Among them, the bionic thumb threshing elements were tailored to match the spiral arrangement of kernels of corn cobs, particularly with a radius of 10 mm and a height of 84 mm. The threshing elements were arranged in a multi-helical staggered pattern, in order to enhance the uniform contact for the low local stress. Specifically, 4 groups of rigid-flexible coupled elements were uniformly distributed at 90°, where 8 elements per group were arranged alternately along two spiral lines (22.5° and 60°); 8 groups of bionic thumb elements were staggered at 45°; and 4 groups of separation plate seats were evenly spaced at 90°. The multi-helical staggered arrangement was also optimized for the better contact between threshing elements and kernels. Key components of the threshing device were theoretically analyzed after optimization. Furthermore, a multi-scale model of flexible corn ears was constructed with the three-layer bonding keys using grid division and coordinate filling, according to the Hertz contact theory and the Discrete Element method (DEM). These three-layer bonding keys were designed to simulate the bonds between kernels, between kernels and cobs, and within cob tissues, respectively. Mechanical calibration was conducted to verify the accuracy of the simulation model and the threshing device. A comparison was performed on threshing in the closed-type threshing drums. Among them, the current mainstream conventional open-type threshing device was adopted the rigid spike-tooth drums and grid concave plates. The ear model was established for the kernel detachment, cob bending, and breakage within the device. Single-factor experiments were conducted to identify the influencing factors on the threshing performance and their key parameter ranges. A Box-Behnken response surface design was adopted to develop the experimental scheme. A systematic investigation was made to explore the influence of the interaction between various factors on the threshing performance using orthogonal experiment and response surface analysis. Meanwhile, bench tests were then conducted with the three factors as the independent variables, while the kernel breakage rate and unthreshed rate as evaluation indicators. The three factors included the drum rotational speed (350-450 r/min), feed rate (7-9 kg/s), and concave clearance (35-45 mm). These ranges were covered the optimal combination of parameters for the boundaries after multi-factor optimization. The test results indicate that the primary influencing factors on the threshing performance were ranked in the descending order of significance: drum speed, concave clearance, and feed rate. The optimal combination of parameters was determined as follows: drum speed of 421.77 r/min, feed rate of 7.80 kg/s, and concave clearance of 40.54 mm. Under this parameter combination, the optimal values of the theoretical kernel breakage rate and unthreshed rate were 3.64% and 1.22%, respectively. In engineering application, the concave clearance was rounded to 40 mm, and the remaining parameters remain at the theoretically optimized values. Test results indicate that under this optimal parameter combination, the closed-type threshing drum device designed in this study exhibits a kernel breakage rate of 3.64% and an unthreshed rate of 1.22%. These values were 1.44 percentage points and 0.08 percentage points lower than those of the conventional open-type threshing devices. This verified that the rigid-flexible coupled structure was fully met the requirements of the synergistic damage-reduction mechanism. There was the energy dissipation via elastic deformation and uniform stress distribution by the bionic curved surface. The structure was also met the threshing performance standards stipulated in the national standard GB/T21962-2020, which was required for a kernel breakage rate lower than 5% and an unthreshed rate lower than 2%. The synergistic threshing device with the coupled rigid-flexible and bionic thumb elements can effectively dissipate the impact energy for the uniform stress distribution, thereby reducing the kernel impact damage. The research findings can provide a theoretical and design reference for the low-damage harvesting equipment of the high-moisture corn. A technical solution can also offer to improve the quality and efficiency of the late-season corn harvesting in the Huang-Huai-Hai region.