Abstract: In order to solve the problems that the longitudinal axial flow threshing device was prone to incomplete separation, high crushing rate, high loss rate and high threshing power consumption when the crop feed amount was large, a 360° threshing and separating device for the combine harvester was designed. The device was mainly composed of a threshing drum, an upper concave plate screen, a lower concave plate screen, return plates and a gap adjusting device for concave plate screen. Through the high-speed photography test, the threshing and separation processes of different areas of the device were analyzed. It was proved that the 360° threshing and separating device could effectively reduce the crushing rate and loss rate of grains through a comparative test of operating effects. In order to study the separation performance of different regions of the 360° separation device, a comparative test of the separation performance of different regions was performed. In order to understand the relationship between the separation performance of the upper half of the concave plate sieve and the operating parameters, a single factor test was performed. The rotating speed of the drum, the angle of the deflector and the clearance of the concave plate were selected as influencing factors. The weight of the separated material from the upper half of the concave plate was selected as the test index. The results showed that the mass of the exudate separated in the range of 0°～180° was obviously higher than that in the range of 181°～360°. In the range of 0°～180°, the most exudates separated from the range of 121°～180°. In the range of 181°～360°, the most exudates separated from the range of 301°～360°. The separating performance of the upper part of the concave plate screen increased with the increase of the rotation speed of the drum, increased with the increase of the angle of the deflector and decreased with the increase of the clearance of the concave plate screen. The research results could provide reference and basis for improving the structure and parameter optimization of the longitudinal axial flow threshing and separating device.