The technique of applying fluidized solid magnetic powder as the dense medium to separate coal was applied n a pilot-scale du separation plant and commercial coal mill in China recently. The nature of density distribution of the air fluidized bed dense medium is an important factor that influences the perormance of the separtor. However, during coal processng, fine coal particles would mix with dense medium of magnetite powder and accumulate in the bed. Its behavior will affect the density of the bed. The density distribution with fine coal partieles was studied in a laboratiory experimental system and in the piolt-scale air fluidized bed dense medium dry separator. The fuctional relation between density of air fluidized bed dense medium and fine coal particle content was fitted, applying the principe of least-squares method with damping factors, and a mathematical model was obtained. After analyzing the trial testing data. the mechanism of density distribution of the bed was studied and expanined.