The smaller pulverized coal fineness is better for the ignition and burnout of coal. And the fineness of pulverized coal directly influences the content of combustible in fly ash and slag. In order to solve the operation problems in Shanxi Xingneng Power Plant, such as slag dropping form dry slag machine, high slag temperature, the outage of slag crusher, and so on, adjustment tests of pulverized coal fineness were carried out for the mill. For example, by adjusting the separator speed of mill to change the fineness of pulverized coal; through keeping the oxygen steady at the furnace outlet to study the influence of pulverized coal fineness on the content of combustible in fly ash and slag, as well as the thermal efficiency of boiler. All these tests could provide a basis for the adjustment of rotary separator in medium speed mill. The results show that when the coal finess controlled within 17% and the evenness index of pulverized coal reaches 1.20, the coarse particles in pulverized coal significantly decrease which is beneficial to coal burnout. After the adjustment of pulverized coal fineness, the amount and temperature of slag decrease, which has improved the safety and economy of units.