In order to analyze on the feasibility of coal seam mining with pressure, No. 3 coal seam floor aquifer, water-resisting floor of some mine in Shanxi Province were analyzed, the mechanical properties of the rock mass and the resistance of the protection layer of the floor were studied to get the changing law of floor resisting water layer with water pressure. The result showed that No. 3 coal seam mining with pressure is safe in completed water resisting layer of K2 limestone water and Ordovician water.