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With the demand of modern agricultural production, corn direct grain harvesting has become the inevitable way to achieve efficient harvest. Cleaning process as an important part of corn direct grain harvesting equipment affects the operation performance of whole machine. Aiming at the problems of high grain loss rate and impurity rate in cleaning mechanism of corn combine harvester, and seasonal influence on traditional test, the method of mathematical modeling, simulation and experiment verification were used to optimize the operation performance and the structure design of cleaning device. Based on cleaning system of CASE 4099 combine harvester, a corn threshing and cleaning test platform was built. The cleaning device mainly included upper sieve, lower sieve, tossing board, tail sieve, crank mechanism, fan, motor and so on. The vibrating sieve selected fish scale sieve and adopted doublelayer asynchronous cleaning operation. The mathematical model of movement process of grain on vibrating sieve was established to obtain the change law of main factors affecting on the cleaning effect, and the change law of the average movement speed and movement distance of grain on vibrating sieve were analyzed considering factors such as inclination angel of vibrating sieve, amplitude of vibrating sieve, rotational speed of crank mechanism driving vibrating sieve, angle between vibrating direction of mixture and sieve surface, angle between wind force of fan and sieve surface. In order to improve the cleaning performance, a vertical adjustable air separated board was designed, mainly including support rod, adjusting rod and regulating handle, the internal flow field velocity distribution could be changed by adjusting inclination angle of air separated board, and optimized the performance of cleaning device. Based on corn threshing and cleaning test platform, the experiment was carried out. The flow field inside cleaning device was divided into three layers by boundary of vibrating sieve surface, each layer selected 20 (4 crosses × 5 columns) wind speed measurement points for data collection. The computational fluid dynamics (CFD) simulation technology was used to determine the optimum inclination angle of air separated board slanted at left or right 18°, in this condition, the wind velocity distribution in flow field was uniform, the wind speed change in vertical direction was the smallest, it was advantageous to cleaning operation. To verify the effect of adjustable air separated board, the test results showed that wind speed at each measurement point was uniform distribution and longitudinal gradient of flow field was small when air separated board slanted at right 18°, which was consistent with simulation results. The orthogonal experiment was carried out with vibrating sieve rotation speed and rotation speed of drive motor of fan as main influencing factors, while grain loss rate and impurity rate as indexes. The results showed that when rotational speed of crank mechanism of vibrating screen was 275r/min and rotation speed of drive motor of fan was 900r/min as the optimal working parameter combination, the loss rate and impurity rate were 1.34% and 1.66%, respectively.