In order to solve the blocking problem of threshing cylinder for 4LZ–3.0 rice combine-harvester caused by the large feeding quantity and the high ratio of output of main product to by-product of the crops, a optimized threshing cylinder with longitudinal single axial flow was designed. The threshing cylinder is mainly composed of feeding spiral, spokes, radial disc, threshing components, and row grass board etc. The feed spiral performance test and optimization test were carried out by using the rotation speed of cylinder, the guide plate lead angle and the threshing clearance as the experiment factors and grain crushing rate and unthreshed rate as the performance evaluation index. A mathematical model of the threshing system was established through regression analysis method to determine the optimal working parameter combination. The experiment shows that the threshing performance will be the best with the cylinder rotation speed of 800 r/min, the guide plate lead angle of 23.7° and the threshing clearance of 20 mm, which could reachthe grain crushing rate of 0.113 % and the unthreshed rate of 0.071%.