Corn grain harvesting has become the development trend of corn harvest in China. However, the grain harvesting mechanization is still backward in Huang-Huai-Hai region for the wheatcorn rotation planting pattern. The grain moisture content of corn is about 30% when harvesting. Its low level of the existing grain harvester is unable to meet the requirement of corn grain harvesting with higher moisture content. The broken grains for mechanical threshing are prone to mildew which even threaten food security. The high breaking rate and high un-threshing rate of the traditional threshing and separating device in Huang-Huai-Hai region have been the choke points of grain harvesting development. In order to solve these problems, the longitudinal axial flow flexible threshing and separating device was designed with the combined threshing element and rollertype combined threshing concave. The rollertype combined threshing concave with sixedged mesh sieve and fish scale threshing rubber roller was designed with different traditional concave plate. To verify the feasibility of low damage harvesting, the test device was built and the working mechanism was introduced in detail. Based on the theoretical analysis of the structure, three parameters, i.e. cylinder speed, transmission ratio between roller and cylinder and threshing clearance were chosen as the influence factors of test experiment. And the breaking rate and unthreshed rate could represent the threshing and separating performance. Reasonable variation rage of various factors was determined through single factor experiment. On this basis, threefactor and threelevel orthogonal experiment was carried out. The results showed that the order of influence factors on the breaking rate and unthreshed rate was both cylinder speed, transmission ratio between roller and cylinder and threshing clearance. The optimal conditions were as below: the cylinder speed was 475r/min, the transmission ratio between roller and cylinder was 1.5 and the threshing clearance was 45mm. Under the above conditions, the breaking rate was 3.76% and the unthreshed rate was 0.52%. The verification test showed that this device had good threshing and separating performance, which can meet the requirement of the national standards in China. The research results can provide the references for low damage corn grain harvesting technology.