Abstract: In order to push for automation of separation process of heavy medium cyclone, following an analysis of the separation mechanism of heavy medium cyclone under the effect of external magnetic field, the influences on separation density of the cyclone operating under the action of either static or rotating magnetic field, produced by the following factor's are investigated:thickness and arrangement of magnetic poles, magnitude of excitation current, magnetic structure, inner diameter and combination mode transposal of coil and position of magnetic system of the static magnetic field; and the rotation direction and speed of rotating magnetic field. As revealed by study results, the use of external magnetic field to make online adjustment and control of the separation density of the 2nd-stage of the cyclone offers a series of distinctive features such as no adverse influence on the working conditions and no blockage of the cyclone; the use of the static magnetic field is an energy-saving enviromentally-friendly and highly efficient technology, which has reached a theoretically matured stage in its research; compared to static magnetic field, the use of the rotating version can lead to further improvement of cyclone's separation density, and benenficial to improve sorting accuracy of fine coal. The use of external magnetic field for online control of separation density of cyclone is of great significance for improvement of separation efficiency and realization of automated separation process of heavy medium cyclone.