Abstract: To tackle the problems encountered in the case of using traditional wet cleaning method, such as difficulty in treatment of coal slurry water and excessively high moisture of products, the varying-diameter pulsed airflow separator independently developed is used for dry separation of low-rank coal, and the dynamic mechanism of the separating process is studied with hydrodynamic and numerical analysis methods. As revealed by study results, compared with the constant airflow, the pulsed airflow can accelerate the movement of particles, and the lighter the particle, a more pronounced periodic cumulative oscillation acceleration effect of the pulsed air flow becomes; with the use of the separator for treating the 6~0 mm low-rank coal, the ash of the cleaned product can be reduced by 2.55~3.28 percentage points with an organic efficiency being up to 91.02%. It is demonstrated that the deceleration effect produced by the varying-diameter structure can lead to much better separation of low-rank coal according to density and hence improved separation accuracy.