Abstract: Based on the theory of solid-liquid two-phase flow, four different flow states of coal particles in the drilling process were studied to address the problems of blockage and sticking caused by poor slag discharge during the ultra-high pressure hydraulic slitting operation for drilling along seam.The force condition of coal slag particles under different flow states was analyzed, and the critical flow velocity of drilling slag discharge was determined to be the critical flow velocity of the moving layer flow state.By analyzing the experimental data of slag production in coal seams with different hardness, the relationship curve between slag production and slitting pressure under different coal seam hardness conditions was obtained.A slitting and slag discharge model based on solid-liquid two-phase flow was established, and the optimal slitting pressure that can meet the maximum efficiency of slitting and smoothly slag discharge was determined.The results show that the slitting and slag discharge based on solid-liquid two-phase flow can effectively avoid the problems such as nozzle spraying, blockage, and gas over-limit during the slitting process while ensuring the slitting efficiency.It can be used to guide the design of slitting pressure parameters on site.