The differential equation of airflow in the inertia deposition-separation chamber of 4ZTL-1800 rice combine harvester threshing prior to cutting was developed based on the theory of hydrodynamics and turbulence. The numerical simulation of the 3-D steady airflow in the inertia deposition-separation chamber was made for the first time by using standard k-ε turbulent differential equations, and the airflow characteristic was analyzed. Based on the numerical calculation results，the inertia deposition-separation chamber was optimized and redesigned for reducing the energy loss of airflow turbulence.