Aiming at the problem of poor harvest performance caused by unstable feed rate for combined harvester, a solution was proposed to adjust threshing clearance parameters to adapt to different feed rate. In order to realize adjustable threshing clearance, a threshing drum with adjustable diameter was designed based on 4LZ-1.0 small combine harvester. The roller consists of a spindle, a gear rod, a clearance adjusting mechanism and a clearance controlling mechanism. The clearance adjustment mechanism is driven by the clearance control mechanism, and the threshing drum diameter is changed to realize the threshing clearance adjustment, and the clearance adjustment range is 10-40 mm. The threshing test was carried out on the test bench, with the feed rate cylinder speed and concave clearance as the influencing factors, and the performance evaluation indexes of unthreshing rate, segregation and separation loss rate and impurity rate as the evaluation indexes. The effect of various factors on the threshing performance of the device was analyzed by regression analysis. According to the analysis of the comprehensive evaluation regression equation, the optimal cylinder speed and concave clearance under different feed rate are obtained. When the feed rate is less than 0.876 kg/s, the cylinder speed should match the low speed of 700 r/min, and when the feed rate is greater than 0.876 kg/s, it should match the high speed of 1 300 r/min. A linear equation of matching threshing clearance is obtained dependent on the feed rate.