A computational fluid dynamics (CFD) model is proposed to describe the multiphase flow in a dense-medium cyclone (DMC). In this model, the volume of fluid (VOF) multiphase model is first used to determine the initial shape and position of the air core, and then the so called mixture model is employed to describe the flows of the medium, coal particles and air, where the turbulence is described by the Reynolds stress model. The validity of the proposed approach is verified by the reasonably good agreement between the measured and calculated results in terms of separation efficiency. On this base, this model is used to quantify the effects of the ratios of spigot to vortex finder diameters (U:O) and medium to coal (M:C) on performance. The results are shown to be generally comparable to those reported in the literature. It reveals that when vortex finder or spigot diameter is varied at the same U:O ratio, the offset and medium split nearly remain the same, however, the coal feed rate and Ep are different under the conditions considered. It is also shown that the fish-hook phenomenon is observed when spigot diameter is equal to or slightly larger than vortex finder diameter, and a normal operation becomes less stable with decreasing U:O ratio. The key phenomena predicted are explained by the calculated inner flows.