Abstract: During the ongoing high-intensity mining of coal, a large amount of coal gasification slag is generated, but its resource utilization rate remains low. The core technology is one of the key technologies for green mining in coal mines, paste filling is facing the problems of high aggregate cost and low solid waste content. Coal gasification slag is used as raw material, and its performance is optimized by mechanical grinding. Combined with X-ray diffractometer, scanning electron microscope, ultra-high-speed intelligent particle size analyzer, etc., the effect of different grinding time on the activity of coal gasification slag is investigated. The results show that with the increase of grinding time, the diffraction peak intensity of coal gasification slag decreases, the particle size distribution increases first and then tends to be gentle, the specific surface area increases, and the area average diameter and volume average diameter decrease. When the grinding time is 20 min, the activation effect of coal gasification slag is the best. At this time, the particle size is concentrated at about 10 μm. Compared to raw coal gasification slag, the specific surface area is increased by 142.1%, the average porosity is increased by 229.54%, the average pore maximum area is decreased by 60.06%, and the average probability entropy is increased by 4.58%. Mechanical grinding improves the activity of coal gasification slag through particle refinement, crystal structure destruction and chemical bond fracture, and provides theoretical basis for the high-value utilization of coal gasification slag as filling material.