Abstract: In order to study the impermeability of the Beigongcun No.2 large fault in No.7 mining area of Dongtan Coal Mine, based on the analysis of geological and hydrogeological conditions in the study area, combined with the steady flow pumping test results of the roof sandstone of No.3 coal seam and 2 Ordovician limestone aquifers exposed by the field hydrogeological drilling, the pumping test data is preliminary analyzed, the hydrogeological parameters of the Ordovician limestone aquifer are obtained, and it is found that there are great differences in water abundance of the Ordovician limestone in different locations.It is considered that there is no hydraulic connection between sandstone aquifer and Ordovician limestone aquifer of No.3 coal seam roof.Based on the quantitative parameters of impermeability obtained from the field pressure and permeability tests in Yanzhou mining area, the geological model of impermeability of the fault is established, the impermeability is calculated by analogy, and compared with the pumping test results, it is further concluded that the fault has good impermeability.The research results have scientific guiding significance for mining in the No.3 coal seam of the study area.