In order to solve the problem of crude plant ash content on the high side, through the analysis and research, the causes of cleaned coal ash content on the high side was found out. At the end of the CSS separation plant in 1 mm of the grade separation effect was poorer, some methods to reduce the desliming sieve seam were proposed. Through reforming practice, a certain extent was formed, which can solve the problem of fine mud pollution, improve coarse coal slime separation effect, and ensure the recovery of coarse coal slime, which provided a possible for the coarse plant dense medium grey back and a direction for further optimization of the system in the future.