Abstract: The trouble with the separation of the 4^# difficult-to-wash coking coal at a low density is that the clean coal is relatively high and unstable in ash and the circulating water is high in concentration. To tackle this problem, a series of technical renovation measures are taken by the washery, such as change of density meter, high-pressure feed pump and coal slime cyclone, fitting percussion device on clean slime sieve bend, addition of rake thickener and tailings filter press, and remoulding of clean slime dewatering process. Practice shows the technical renovation has achieved the desired effects as evidenced by the drop of clean coal ash to ＜ 8.5%, a rise of ash stability rate up to 95% and a decrease of circulating water concentration to a level of ＜ 0.5 g/L.