Abstract: In face of the challenge of variation of raw coal property, and thanks to the availability of the technologies accumulated over years, the H.M. cycloning technology has seen rapid development and has now become the primary coal preparation equipment. Following a description of the basic structural make-up, types and working principle of H.M. cyclones, the paper goes to elaborate on the stability of heavy medium suspension and the theory involved in heavy medium separation of coal through flow field test and simulation. At the end of the paper, an analysis is made of the problems encountered in the application of the technology as well as the prospect of its future development.