Abstract: Abstract: Threshing has been one of the most fundamental steps to separate the grains from the harvested crop. An adjustment device of threshing clearance can greatly contribute to matching the feeding amount and working parameters in a combine harvester, in order to reduce the damage or loss rate of clean grain in the field. Two parameters are often used to adjust the threshing clearance at present, including the diameter of the cylinder and the position of the concave. In terms of cylinder diameter, there is a relatively larger structure of the device to control the expansion and contraction of the threshing teeth, leading to high power consumption and production cost. Therefore, the vertical adjusting concave has been commonly used to adjust the position of the concave in the combine so far. However, the vertical concave body can normally move up and down to result in the limited threshing clearance at the bottom of the threshing cylinder in the device. In this study, a symmetrically adjustable concave screen was designed to adjust the threshing clearance at the bottom of the cylinder in both sides. Among them, a hinged shaft was welded between the concave body and the upper cover plate that was installed on the frame, and an electric cylinder was positioned on the screen body. Specifically, a mobile phone in Bluetooth mode was utilized to extend or contract the electric cylinder during operation, further to drive the concave body for the rotation around the articulated shaft, and finally to quickly adjust the threshing clearance at the bottom in both sides of the threshing cylinder. A circular curve function was also selected to calculate the cross-sectional area of threshing clearance for the two concaves in the rectangular coordinate system before and after adjusting the threshing clearance. After that, a comparison was made to determine the working performance of the new symmetrically adjustable concave and vertical one on threshing clearance. The results showed that the symmetrically adjustable concave body was realized to bilaterally adjust the threshing clearance at the bottom at both sides of the cylinder. The cross-sectional area of the threshing clearance also varied greater than that of the vertical one. The grain logistics density was directly proportional to the rubbing effect inside the grain logistics. Once the feeding amount decreased, the threshing clearance was reduced to improve the grain logistics density for the internal rubbing effect and the low rate of unthreshed grains. Once the feeding amount increased, the threshing clearance needed to increase for the low grain logistics density, where the internal rubbing effect and the breakage rate of grain were reduced to directly tailor the density of grain flow via the cross-sectional area of threshing clearance. Correspondingly, the performance of the symmetrically adjustable concave was better than that of the vertical one, according to the cross-sectional area of threshing clearance. A field experiment showed that an optimal combination of parameters was achieved, where the forward speed of 3.3 km/h, the cylinder speed ratio of 500:650, the threshing clearance of 19 mm in the threshing section, and 15mm in the separation section. The optimal performance was obtained in this case, where the breakage rate of grain was 2.69%, the rate of unthreshed grains was 0.57%, and the loss rate of separation grains was 0.13%, indicating the higher reduction by 0.46%, 0.71%, and 0.55%, respectively, compared with the vertical one. The indexes of the symmetrically adjustable concave were also better than before. The findings can provide a strong reference for the differential threshing and separation in the adaptive regulation system of soybean combine harvester.