A novel inclined tapered diameter separation bed (ITDSB) was designed for the beneficiation of coarse coal slime. Computational fluid dynamics (CFD) coupled with the discrete element method (DEM) was explored to study particle separation and the effect of underscreen water on particle movements over time. Results show that coal particles and gangue particles have disparate movements. Coal particles are drifted by the force of water flow and have the analogous trend of movements as water flow, while gangue particles sink towards the lower part of the bed under the effect of the gravity. Secondary separation was achieved through forcing particles to bounce back into the flow field and benefited from the invention and application of underscreen holes. The three factor orthogonal experiments of separating coarse coal slime (3-0.5mm) have been performed. The obtained clean coals and tailings were tested and further analyzed with an X-ray fluorescence spectrometer (XRF) and scanning electron microscope (SEM). XRF and SEM results show that gangue minerals were discarded significantly after the separation. Orthogonal experimental results reveal that with reasonable combination of various levels of factors, the average combustible recovery is approximately 60%, which shows that the inclined tapered diameter separation bed is efficient equipment for coarse coal slime beneficiation. Secondary separation and the effect of underscreen water were simultaneously verified as efficient methods in particle separation processes.