Abstract: Abstract: In order to optimize the length of longitudinal-flow threshing cylinder of combine harvester, in this paper the length-adjustable longitudinal-flow threshing cylinder was designed and used to conduct rice threshing and separating performance experiments. The grain distribution rules and distribution equation under the grid concave were developed. The length of longitudinal-flow threshing cylinder was optimized by calculating the optimal deflector helical angle of cylinder cover. The optimal length of the longitudinal-flow threshing cylinder was used to conduct rice threshing and separation performance confirmatory test. The result showed that when the feeding rate was 7 kg/s, adapted deflector helical angle of 7.64°, the optimal length of the longitudinal-flow threshing cylinder was 3 159.77 mm, and then the un-separated grain loss ratio could be controlled at the range from 0.2 % to 0.3%. The verification test result was about 0.29%. This study may provide a basis and method for design the length of longitudinal- flow threshing cylinder of combine harvester.