In the process of maize grain direct harvest, the threshing cylinder speed should be matched with the feed rate of combine harvester, so that the best effect of the operation can be achieved. In order to obtain the optimal ranges of the threshing cylinder speed of maize combine harvester at different feed rates, a longitudinal axial flow maize threshing and separating device was designed and developed, whose components and parts can be changed easily. It was also convenient to adjust the structure parameters and working parameters. The threshing cylinder speed and the feed rate were selected as the influence factors, and the maize kernel damaged rate and un-threshed rate were selected as the performance indexes in the experiments. The test results showed that with the increase of threshing cylinder speed, the maize kernel damaged rate was increased and the un-threshed rate was decreased. At the same threshing cylinder speed, with the increase of the feed rate, the maize kernel damaged rate and the un-threshed rate were increased slightly. Through the bench tests, regression analysis and single variable solution, the optimal ranges of the cylinder speed at different feed rates of the threshing and separating device were determined. The optimal range of cylinder speed was 254~486r/min at the feed rate of 8kg/s. The optimal range of cylinder speed was 278~466r/min at the feed rate of 10kg/s. The optimal range of cylinder speed was 313~445r/min at the feed rate of 12kg/s. Under all the conditions above, the maize kernel damaged rate was less than 5%, and the un-threshed rate was less than 2%, which reached the requirements of the national standards. The research result of the threshing and separating device provided reference for design of maize grain harvester, and the research on adaptive model of the feed rate and threshing cylinder speed.