Abstract: In order to clarify the coal -water mixture in the hydraulic slotting slag discharge process in the drilling hole movement rules and drilling cinder bearing capacity, to avoid the occurrence of plugging holes, holding drilling and other phenomena due to the slit falling coal is greater than the drilling cinder bearing capacity, first of all, we need to grasp the morphology characteristics of the coal slag particles. The OCCHIO ZEPHYRESR2 dynamic image particle size and particle shape Instrument was used to test the particle size and particle shape of the coal slag particles in the hydraulic slotting of four working conditions in the underground coal mine. The particle size characteristics were characterized by the Feret diameter minimum, and the particle shape characteristics were characterized by the circularity and the irregularity indexes. The results show that: in terms of the number of particles, the particle size of hydraulic slotting cinder is mainly distributed in the range of 100 μm to 350 μm, and in terms of the volume of particles, the particle size of the cinder is mainly distributed in the range of 500 μm to 4 000 μm. The circularity of cinder particles is mainly distributed in the range of 0. 6 to 0. 8, and the irregularity are mainly distributed in the range of 0. 45 to 0. 65. Hard cinder particle size is larger than soft coal, and cinder particle size is larger in the first 5 min than in the last 5 min of a single slit, which provides basic parameters for the movement law of coalwater in the borehole, and it has important reference and significance for the adjustment and optimization of the relevant parameters of hydraulic slotting.