Abstract: Based on the analyzing the advantages and disadvantages of decreasing the low limit of particle size of mechanical refuse discharging,presented the main principle of determining the low limit of particle size of refuse discharge,and according to that principle,researched the low limit of particle size of refuse discharge in coal preparation plants of Datun mining area,finally concluded that in Datun mining area,the low limit of particle size can be determined as 25 mm,and taking the shallow bath dense medium separator for mechanical refuse discharging.