Abstract: The threshing and separating device of combine harvester is an important working part of combine harvester. When the structure and parameters of the threshing and separating device were changed, it will affect the distribution law of the threshing and separating device, and the consistency between the distribution law of the threshing and separating device and the flow velocity distribution law of the airflow field will affect the cleaning quality. However, the existing threshing and separation device has limited ability to adjust the distribution of the separated substances under different types and working parameters, resulting in high crushing rate, loss rate and other indicators. Therefore, the purpose of this study is to improve the consistency of the distribution law of threshing and cleaning air flow. An independent adjustable concave screen with four concave screens can independently adjust the corresponding threshing clearance was designed. The concave screen was driven by an electric cylinder to rotate around the hinge shaft to adjust the threshing clearance. At the same time, because a pressure sensor is installed in the middle conveyor of the harvester, The threshing clearance adjustment mode can be switched to the automatic adjustment mode through the mobile phone. At this time, the corresponding feeding amount and threshing clearance can be calculated by detecting the pressure of crops on the bottom of the intermediate device, so as to realize the automatic adjustment of threshing clearance. At the same time, in order to study the adaptability of the independently adjustable concave screen to different crops and determine the optimal parameter combination for threshing soybean, a threshing separation test-bed was built according to the structure of the threshing separation device and cleaning device of the combine harvester commonly used in Southern China. Firstly, the velocity distribution of air flow field in the cleaning room was measured, and then the mass distribution of soybean, rice and maize threshings were measured respectively when the threshing gap was symmetrical. It was found that the velocity of air flow field was symmetrically distributed along the axis of the threshing cylinder, but the three crops' threshings were high on one side and low on the other, or in the shape of a saddle. Then, the threshing quality of symmetrical and asymmetrical threshing gap was compared. It was found that increasing the threshing gap on the side with lower threshing quality could improve the threshing quality to a certain extent, and the conclusion was suitable for the above three crops. Finally, due to the low rate of mechanized soybean harvest and the lack of special soybean combine harvesters in Southern China, in order to further improve the quality of mechanized soybean harvest in Southern China, it is necessary to optimize the parameters of the new threshing and separation device. The bench test and field verification test show that when the feeding rate was 3.4 kg/s, the length ratio of the front and rear cylinders was 1:2, the rotation speed of the front cylinder was 443 r/min, and the rotation speed difference of the cylinder was 93 r/min, When the left front threshing clearance was 22.7 mm, the front threshing clearance difference was 5.4 mm, the left rear threshing clearance was 18.6 mm, and the rear threshing clearance difference was 3 mm, the quality of mechanical harvesting operation was the best. At this time, the crushing rate was 2.64%, the loss rate was 1.12%, and the impurity content was 1.97%, which was 0.56%, 0.53%, 0.65% lower than the existing threshing and separation device. The asymmetric layout of threshing clearance can improve the work quality better than the symmetrical layout. At the same time, the same test method was used to conduct performance tests and verification tests with corn and rice as test materials respectively, and the results showed that the machine harvesting performance indexes have been improved, and the machine harvesting performance indexes have been improved. The research results can provide reference for improving the adaptability of combine harvester to different crops.