Taking the horizontal axial threshing unit of the miniature wheat combine for hills and mountains as the research object, the speed of the roller, concave gap, and angle of the guide plates were researched in the orthogonal mesh size experiment under the rated capacity feed. The roller speed, concave gap, angle of the guide plates were researched in the regression testing. Then by using the DPS and other software, the statistical analysis of test data was got. The influence curve of the roller speed, the concave gap, the angle of the guide plates, mesh size to the ratio of the long-stem, the grain breakage, entrainment, threshing without a net were obtained. The regression equation was built. The optimum operating point for the mini-combine was the mesh size of 14 mm, the angle of guide plate of 10°, the concave clearance of 16 mm and roller speed of 660 r/min. Accordingly, long-stem rate was 3.1%, impurity rate was 39.5%, entrainment rate was 0.6%, the rate of threshing without a net was 0.01%, and grain breakage rate was 0.01%.