Abstract: To solve the problem of mining under safe water pressure of aquifer with high water pressure and thin traditional water-resisting layer in lower coal seams of Zhaozhuang coal mine in Shanxi Jincheng Mining Group, by adopting hydrogeological drilling, pumping test, water injection test, in-situ stress measurement and indoor test, based on the comprehensive study on the lithology, filling situation of fractured karst, drilling leakage, rock mechanical index and permeability test in the top of Ordovician limestone, the relative water-resisting property of rock strata in Fengfeng?Formation in the top of Ordovician limestone which is the floor of lower 15# coal seam is demonstrated. The results show that:1) the lithology of the upper Fengfeng Formation existed in the top of underlying Ordovician as floor of 15# coal seam in Zhaozhuang coal mine is dark grey, dense, hard and thick-layer limestone with less dolomitic limestone and marl, which has high strength and a large number of irregular filled fractures and constitutes strata of poor permeability. 2) There is a layer of relative aquifuge with at least 30 meters thick existing in the top of Fengfeng?Formation of Ordovician limestone. The thickness of underlying water-resisting layer as floor of 15# coal seam is between 38.75~67.25 meters and the average thickness is 57 meters, which has better water-resisting property. 3) The risk assessment of water-inrush during mining lower 15# coal seam in Zhaozhuang coal mine is performed by the method of water-inrush coefficient, which shows that the risk of water-inrush can be greatly reduced by applying the relative water-resisting layer in the top of Ordovician, and then a great amount of coal resources will be released.