Abstract: The coefficient of water-invasion is usually used as judgment criterion when the coal seam suffered from Ordovician limestone water invasion.This paper adopts small-size indoor experiment of hydraulic fracturing to measure the coefficient of water-resisting of coal seam floor.The coefficient of water-resisting is used to evaluate the dangerousness of Ordovician limestone water invasion in the exploitation of lower coal seam(Coal16、Coal17),the first mining area,Xinglongzhuang Mine.Resultsindicated that this method is of feasibility for evaluating dangerousness of Ordovician limestone water invasion.