Abstract: Rape plant is high and large with branches implicated, fried pods, large mechanized harvesting loss in harvest period. In order to reduce the rape harvest loss of combine harvester, the rape cutter of 4LZ-4.0 grain combine harvester was redesigned by lengthening the cutter 450 mm, installing 1 150 mm double-acting cutter half-distance stroke vertical side cutting knife, adjusting the reel wheel center position, reducing reel wheel speed to 12-21 r/min, heightening 400 mm side rear wall and other methods. The field harvest loss comparative experiment of three rape planting modes were carried out on the prototype, the results showed that the rape cutter loss rate was 4.44%, the total loss rate was 6.60%（＜8.00%）, 45% and 36% lower than before improvement respectively. It has proved that the design has certain production practice reference value for the harvester rape cutter loss rate.