Abstract: To solve the problem of low ash content of middlings and refuse in dense medium system in Wuobei coal preparation plant,taken the measure of adding the coarse particle size medium into the dense medium separation system. After adding the coarse particle medium into the dense medium system, while other conditions unchanged,the ash content of middlings and refuse increased by 5% and 2. 5% respectively,that initially solved the problem of low ash content of middlings in dense medium system, increased the recovery rate of middlings.