The invention discloses a winnowing device on a corn harvester, wherein the cleanness of corn kernels can be improved. Threshing chambers are fixedly arranged on the frame, a threshing roller arranged in each of the threshing rooms is rotatably connected with the frame, a suction opening is formed in the threshing chamber, an exhaust fan is arranged in the suction opening, a transmission wheel on the exhaust fan is connected with a transmission wheel fixedly arranged in the end part of the threshing roller through a conveyer belt, and the suction opening on each of the threshing chambers is formed in the upper part of the threshing chamber and is positioned in the middle rear part of the threshing chamber. The exhaust fan is a crossflow type ventilator. Two threshing chambers are arranged and the suction opening is positioned at the middle connection of the two threshing chambers. As the suction opening is formed above the middle rear part of the threshing chambers, the probability of sucking impurities such as fine wool matters and corn bran is increased, so that the cleanness of the kernels can be guaranteed. In addition, an original centrifugal fan is replaced by the crossflow type ventilator, so that the winnowing effect is improved, thereby favorably improving the cleanness of the kernels.