Abstract: In order to solve the problem that coal seam can be drilled only upward but not downward in process of uncovering coal of crossheading and shaft, a core-drilling device of dual-core and single-switch was used to do core-drilling test in shafts of some mine in Shanxi province and crossheading tunnel of Licun Coal Mine of Lu'an Group. Main influencing factor of failing in down core-drilling was summarized, which was residual water in borehole. Some measures were adopted to drain the residual water, including grouting, pumping water and deslagging with pressure air in whole core-drilling process, so as to realize complete core-drilling of coal seam. It was pointed out that the sign of successful core-drilling is that a lot of dry coal dust bursts forth form drilling hole in drilling process of core-drilling device in coal seam.