In order to solve the problem of high broken grains caused by high moisture content in corn harvest in North China, a drum with gradually changing diameter was designed. The diameter in the front of the drum was gradually increased until it was the same diameter as the threshing separation section. By improving the ear holding capacity of the drum in the front section, the flexible contact between the ears was enhanced, which effectively lossening the connection between the kernels and the connection between the kernels and cob. The ears were easily threshed, and the separation of the grains and materials other than grain was realized quickly. The threshing speed was effectively improved, and the grain breakage rate was reduced. The stress between ears and threshing element were analyzed, and the influence of taper on the stress of ears ware determined. Based on the dynamic simulation, the contact force between the ears and the threshing elements, the contact frequency between the ears and the contact frequency between the ears and the threshing device were analyzed. The results showed that the drum with gradually changing diameter can increase the contact frequency between the ears and reduce the direct contact between the ears and the threshing element, that was, the contact and rubbing action between the ears in the drum with gradually changing diameter was stronge. The orthogonal test was carried out with roller speed, concave clearance, and grain moisture content as experimental factors. The optimal combination was determined as follows: grain moisture content was 26%, roller speed was 350 r/min, concave clearance was 50 mm. At this time, the grain breakage rate was 4.13%, and the percentage of unthreshed grains was 0.34%. When the grain moisture content was 27%, a comparative threshing test was carried out with the equal diameter drum. According to the integrity of the grain, the damaged grain was divided into completely broken grain, crack grain, skin damage grain, and broken grain. The total broken rate of the grain was 4.64%, which was 19.16% lower than the 5.74% of the equal diameter drum. The proportion of completely broken grain, crack grain, and broken grain in damaged grain was decreased significantly. The percentage of unthreshed grains was 0.42%, which was 51.72% lower than the 0.87% of the equal diameter drum. It was proved that the drum with gradually changing diameter can effectively reduce the rates of broken grains and the rates of unthreshed grains.