Abstract: There is a strong trend of sedimentation in low density heavy medium suspension, which may worsen the effect of cyclone separation and enrichment of vitrinite in dense medium cyclone (DMC). In this study, low ash slime, montmorillonite and their mixed composite slime were used as non-magnetic materials to reveal their effects on the cyclone stability and actual separation. The results show that: 1) with the increase of non-magnetic content, the underflow concentration, overflow clarity, concentration efficiency and underflow density difference of the suspension decrease, and the stability increases. And the addition of composite slime has better stability. 2) the concentration efficiency and overflow clarity of suspension with montmorillonite and magnetite powder are higher than those without non-magnetic substance, indicating that the system is more unstable. 3) the target of concentrate with ash content less than 3% and vitrinite content higher than 75% can be obtained by using suspension with magnetic content of 30% and 40%. The lowest possible deviation E value of separation is 0.034g/cm3, but when its content is more than 50%, or the suspension with only low ash slime or montmorillonite cannot meet this requirement. 4) although the addition of too much non-magnetic material can further improve the stability of the suspension, it can also hinder the migration of the selected coal particles in the DMC and make the separation effect worse. therefore, there is an optimal value of non-magnetic content in the suspension.