Abstract: In order to investigate the reason why the coal slime water with high clay minerals content was difficult to treat, taking depth de-ashing flotation clean co al as the research object, the influence of kaolinite, montmorillonite and llite on the surface properties of the coal slime was analyzed through Scanning Electron Micros cope and Zeta potential instrument.The Scanning Electron Microscope results showed that kaolinite and ilite in slurry had many coarse particles with bad distribution W hich was against to the sedimentation, while montmorillonite was easy to disperse more evenly after softening and degradation.Zeta potential results showed that in Co mparison with kaolinite and ilitle, the coal sample added montmorillonite improves the Zeta potential value which lead to stable suspension of coal slime particles again st the sedimentation.The results of the surface morphology and Zeta potential showed that montmorillonite was the most important clay minerals which deteriorated slur ry settlement.