Abstract: Abstract: Ear picking and grain harvesting are the 2 kinds of corn mechanized harvesting. Grain harvesting can complete harvesting, threshing and cleaning process at one time, which reduces operation process and improves working efficiency. It is the development trend of China corn harvest. The planting pattern in Huang-Huai-Hai region is wheat-corn rotation. The grain moisture content of corn is about 30%-35% when harvesting. This causes high breaking rate and un-threshing rate and restricts the development of corn grain harvest. There were many current studies on corn threshing process at home and abroad. But most studies were on small corn threshing machine. This article analyzed the factors affecting breaking rate and un-threshing rate, which provided a theoretical basis for research of large corn grain harvest machine. Based on analysis of the structure characteristics of the existing threshing cylinder, this article designed the combined axial flow corn threshing cylinder. Based on analysis of threshing process characteristics, radial feed and radial exhaust style cylinder was selected. According to calculation, the threshing cylinder designed had a diameter of 650mm and a length of 2800mm. The cylinder chose rasp bars and nail teeth as threshing elements. In order to make the material in the cylinder have a backward transmission capacity, the rasp bars and nail teeth were arranged in 4 heads withthe pitch of 4800mm. The test bed adopted grid and punched combined concave. Two spiral guide bars were arranged at the end of the inner wall of the cover to control the speed of material movement. The spiral angle of the guide bar was 35°, and the diameter was 8mm. In October 2016, using the threshing test bed, the threshing experiment was completed at the experiment base of Shandong Academy of Agricultural Machinery Sciences(SAAMS) in Zhangqiu District,Jinan City, ShandongProvince. Experiment subject was summer corn in mature period in Shandong with the moisture content of 32%-34%. The experiment was carried out according to GB/T 21961-2008 Test method for maize combine harvester and GB/T 21962-2008 Technical requirements for maize combine harvester. The cylinder speed, cylinder angle and concave clearance were chosen as experiment factors, and the breaking rate and un-threshing rate as main test indices. In each experiment, 150 corn ears were fed, and the experiment of each group was repeated 3 times. In order to determine reasonable range of each factor, single factor experiments of the 3 factors were firstly carried out. The single factor experiments showed that with the increase of cylinder speed, the lowest breaking rate was 5.52% when cylinder speed was 450~500 r/min; the un-threshing rate decreased and the lowest un-threshing rate was 0.2%when the cylinder speed exceeded 350 r/min. The breaking rate decreased while the un-breaking rate increased with the increase of concave tolerance and the lowest breaking rate was 3.35%, the lowest un-threshing rate was 0.12%. The breaking rate and un-threshing rate were both decreased with the increase of cylinder angle and the lowest breaking rate was 2.88%, the lowest un-threshing rate was 0.06%. On the basis of single factor experiment, this article chose cylinder speed, cylinder angle and concave clearanceas investigation factors and carried out orthogonal experiment with 3 levels of each factor. By using SAS (statistical analysis system) software and Duncan's method, main sequence and significance level of the factors were tested, and the optimal parameter combination was found out. The orthogonal experiment showed that the sequence of the factors affecting breaking rate and un-threshing rate was cylinder speed, cylinder angle and concave clearance. The factor combination obtaining the lowest breaking rate and un-threshing rate was cylinder speed of 430r/min, cylinder angle of 6° and concave clearanceof 55mm. With this factor combination, the breaking rate was 2.96% and the un-threshing rate was 0.19%.