Abstract: Grain gravity, buoyancy and gas drag were taken into consideration in burden trajectory calculation and the movement trajectory of a particle in chute and freeboard was computed. The influencing laws of gas drag on the landing point were analyzed according to the discussion about the variation of charging radius with different burdens in their range of grain diameter and the percent of gas drag. It is shown that the effect of gas drag should be considered for the precise calculation of burden trajectory, the landing points of particles are not the same because of their different density, size and shape factor, and burden distribution is influenced by the distribution of top gas flow.