A new inertia deposition-separation chamber was adopted in 4ZTL-1800 combine harvester threshing prior to cutting to separate grain from airflow, however, particle trajectory, its separating and depositing process in the grain inertia deposition-separation chamber have not been studied so far. Trajectory of particle in the combine inertia deposition-separation chamber could be tracked by discrete phase model of Lagrange. The differential equation of grain moving in the inertia deposition-separation chamber was developed based on Newton's Second Law. The numerical simulation about particle trajectory, its' separating and depositing process in the grain inertia deposition-separation chamber were carried out for the first time. Furthermore, the effect of inlet airflow velocity and initial velocity of particle to efficiency of particles deposition and separation were also simulated and analyzed, which is beneficial to structure optimization design of the inertia deposition-separation chamber.