Abstract: In order to improve the separation effect of the high muddy and difficult floated slime ,taking the difficult floated slime in Xizhao Coal Preparation Plant as a study object,with different structure parameters( cone structure of the cyclone,inner diameter of the overflow pipe and diameter of underflow pipe) and the feeding pre ssure,a small diameter( 50 mm) new mode water medium cyclone was applied to the gravity separation experiment. The results showed that the ash content of underflo W in the water medium cyclone was about 80%,the ash content of + 0.045 mm coal in the overflow was only 6% and thus a high efficient separation of the difficult float ed slime could be realized. With the series connection between the new water medium cyclone and the hydraulic cyclone, the-0.045 mm particle material in the overflow could be effectively removed after the separation and the clean coal with an ash content of 18%. Thus the new water medium cyclone and the hydraulic cyclone would b e effective access to separate the difficult floated slime.