In view of the loss of maize cleaned out of maize harvester, models of maize mixture were founded by using EDEM software. Computational fluid dynamics and discrete element method (CFDDEM) were coupled to simulate the motion of maize in cross airandscreen cleaning device. The distribution of airflow and the trajectories and velocities of maize moving in the device were obtained, which reveal the work mechanism of the device. The results also showed that the debris of maize only included maize cob and main concentrated in the middle and at the rear of the device. And the performance of the device cleaning debris was enhanced with rises of inlet airflow velocities in the device, but the loss of maize was increased. The experimental results and simulation results consisted well. In order to meet the performance requirement of maize harvester, the loss rate of maize should be lower than 2%, the inlet velocity of airflow was ascertained and should be lower than 11.72m/s under the condition of the normal feed quantity of maize mixture of 3kg/s. Based on this, the rotating speed of the cross fan in maize harvester should be lower than 911r/min.