Harvesting grain directly has been the development trend of modern maize combine harvester. To satisfy the demand of harvesting grain straightly type maize combine harvester for large feeding rate, low damage rate, high threshing rate, the longitudinal axial flow threshing and separating device was designed. It could also increase the separating rate of grain and miscellaneous, and reduce the burden of cleaning device. Threshing gap was adjustable in the range of 30 to 60mm, which reduced the incidence of jam faults effectively in the threshing process. Spiral blade on feeder, threshing rasp-bar and separating nail were replaceable, which improved the efficiency for threshing and separating.