The axial flow threshing roller of 4LZ-3 combine harvester was divided into three sectors: feeding sector, threshing sector and separation sector. The movement of cereal passing through each sector was analyzed based on reasonable assumptions. Subsequently, formulas of absolute velocity, relative velocity and axial direction velocity were deduced considering the friction force. The results show that the helix angle of feeding sector should be smaller than the complementary angle of friction angle between cereal and helix lamina, helix angle of threshing sector and separation sector should be smaller than friction angle between cereal and leading lathing respectively.