Abstract: In the drilling process of soft and outburst coal seam, the rib drill pipes and central ventilation are generally used to discharge slag. Due to the limitations of existing slag removal technology, the coal slag blockage often occurs and seriously affects the depth and porosity of drilling. Taking the horizontal drilling of rib drill pipe as an example, the force condition and movement law of coal slag in the process of slag discharge are analyzed. When the rate of slag production is greater than the rate of slag discharge, the discharge channel of rib drill pipe will be blocked. The critical slag generation rate of slag discharge channel blocking for ?准73 mm rib drill pipe is calculated by the established formula of slag generation rate and slag discharge rate. The results of laboratory verification show that it is feasible to control the drilling speed of rib drill pipe to avoid the blocking of slag discharge channel.