A theory of the dry separation of coal by density difference is propose. The mecjamos, mad efficiency of separation of coal particles in an air dense medium fluidized bed (ADMFB) according to bed density and the density distribution in the fluidized bde is studied. The results show that the uniform a nd stable fluidized bed can be formaed at specific conitions. The maximum of relative errors of bde densities is only 2%, and the raw coal of 50 by 6mm size can be separated effctively with a high recovery of elean coal and a separation precision characterized by Ep value of 0.03. Only a small amount of misplacement of material between the light an heavy products was found. The view of "misplacing effects of viscosity" and "misplacing effects of motion" was proposed to explain the experimenta results satisfactorily.