Abstract: In view of low slag discharge efficiency and hole-forming difficulty problem of pneumatic drilling machine in coal mine roadway floor, multiphase flow pattern change in drill-hole during slag discharge process has been analyzed based on two-phase flow pattern change. Meanwhile, reasons that cause hole-forming difficulty have been analyzed from the point of moving force path of drilling slag. The results show that two-phase pattern change, such as “static - bubble flow - slug flow - annular fog flow” were successively experienced in the hole, about 75% of drilling slag is discharged at a faster rate during the bubble and slug flow stages, while the remaining drilling slag should be discharged slowly in the stage of annular fog flow with low efficiency; external expansion of drill-hole caused by water erosion is the main factor leading to drilling failure. Based on the engineering practice of -250 cross-hole in Yixin Coal Mine, the “casing beam” deslagging method is proposed correspondingly and the original drilling technique is optimized, and the field application effect is good.