Abstract: In this paper,in order to study the effect of various ultra-fine grinding modes on ultra-clean coal separation effect,taking the different characteristics of ultra-fine grinding methods into consideration,choosing stirring mill,air mill,colloid mill and ball mill for comparisons,the coal samples were processed by four ultra-fine grinding to product ultra-clean coal. The results show that the flotation could obtain the highest yield of 84． 55% and the lowest ash of 0． 83 by using colloid mill,thus the colloid mill is more conducive to improving the ultra-clean coal separation effect. In order to study the mechanism of the effect of grinding on the separation efficiency of ultra-clean coal,the effect of ultra-fine grinding on the geometrical characteristics of coal particles was obtained through the study of size distribution and SEM. The results show that when the coal was processed by colloid mill,its size distribution was uniform,refine- ment was detailed and the fine agglomerates were avoided,the coal particle surface had no obvious fractures,a lot of ir- regularities occurred and the sharp corners were blunted. The effect of ultra-fine grinding on the surface chemical prop- erties of coal particles was obtained through the study of AFM,FTIR and wet heat curves. After the processing of colloid mill,the AFM results show that the surface roughness (Rq ) of the coal sample had the highest value of 2. 96 nm,the change of height was the largest and more functional groups were exposed,that meant the coal sample was dis- sociated effectively. The results of FTIR show that the contents of functional groups on the surface of coal sample were changed significantly,the ratio of hydrophobic functional groups to hydrophilic functional groups absorption peak area was the biggest with a value 0. 084,that meant,the organic matters and the inorganic matters in coal samples were dis- sociated efficiently,and the hydrophobicity of coal samples were improved. The wetting and heat curves showed that the coal sample had a stronger effect with the non-polar oil,the value of wetting heat was 1. 384 J / kg,which was beneficial for fine particles to form floccules with tight structure and high selectivity. The formation of a compact,stable and se- lective floccule can effectively improve the separation efficiency of ultra-clean coals.