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In order to improve cleaning performance of air-and-screen cleaning device in maize harvester, the mechanism of throwing impurity, which was installed in the upper space of cleaning screen to make maize mixture disperse in the horizontal direction and layer in the vertical direction, was invented to meet requirements of the increasing feeding quantity. The CFD-DEM coupling method was used to simulate the delamination of maize mixture with the help of mechanism of throwing impurity in the vertical direction. The experimental factors were the numbers of raking finger in circumferential direction, the rotation radii of raking finger, the spacing between fingers axially and the angular velocity of roll, and the performance indexes were the horizontal displacement of impurity threw and the overlapping displacement of maize mixture in the horizontal direction. The quadratic orthogonal rotational-combinational simulation test was designed. The test data were analyzed by the response surface method, and the regression mathematical models were multi-objectively optimized by using Design-Expert. The results showed that the numbers of raking finger in the circumferential direction had the most significant effect on the horizontal displacement of impurity threw, followed by the angular velocity of roll, then the spacing between fingers axially, finally the rotation radii of raking finger and the numbers of raking finger in the circumferential direction had the most significant effect on the overlapping displacement of maize mixture in the horizontal direction, followed by the spacing between fingers axially, then the angular velocity of roll, finally the rotation radii of raking finger. The optimized parameters of the mechanism of throwing impurity were the numbers of raking finger in the circumferential direction, which was 8, the rotation radii of raking finger of 80.18mm, the spacing between fingers axially of 12.44mm and the angular velocity of roll of 15.41rad/s. Under the condition of inlet velocities of 12.8m/s and direction angle of the inlet of 25°， when the feed quantity of maize mixture at the entrance of cleaning device was 5~7kg/s,the cleaning rate of maize with mechanism of throwing impurity in cleaning device was 97.20%~98.74% and the loss rate of maize was 1.65%~1.82%, which can meet the requirements of technical specification for quality evaluation of screening under the condition of large feed quantity.