Abstract: The under-group coal seams of Gujiao mining area is in a relatively dangerous area of water inrush in someplace.The possibility of water inrush is greater in tectonic development area.This paper analyzes the pore structures and vertical characteristics of Ordovician limestone through mercury injection experiment,and combining the water location exposed by drilling in pumping test,the existence and thickness of the key-aquifuge are determined.The research shows that upper and lower segment limestone of Fengfeng group are mainly very small and super capillary pores which can be taken as key-aquifuge.The large acreage of under-group coal seams suffered by water pressure threat are liberated by key-aquifuge.