Abstract: In order to reduce the ash content of flotation concentrate in Zhaogezhuang colliery coal preparation plant,take the coal slurry in this plant as study sample,and use the sodium hexametaphosphate,fluorine sodium fluoride and sodium silicate as the depressing to make ash reduction test for coal slurry in the plant.The test result showed that under the experimental conditions,when taking the modulus of 2.4 sodium silicate as depressing agent with the dosage of 500 g/t,the flotation effect was the best;sodium silicate has a certain depressing effect on ＜0.075 mm size fraction fine particles.With increasing high ash fine coal slurry quantity in flotation process,the application of depressing agent in flotation will be used more and more widely.