Modified k—εturbulent differential equations and discrete phase model of Lagrange in Fluent software were used to simulate grain moving in the separation and cleaning chamber in 4ZTL—1800 grain—wheat combine harvester threshing prior to cutting. The grain trajectories, the laws of grain particles separating and depositing, debris including chaff and short straw being cleaned in the chamber were obtained. Furthermore, the relationships between inlet airflow velocity and grain mass flow respectively and grain cleaning rate were also simulated and analyzed, which are beneficial to increase cleaning rate of grain and to decrease the power consume of the machine.