Abstract: The Ordovician water system has a serious impact on the safe mining of coal in Sima coal mine.In order to ensure the safe mining of the lower layer of coal, this paper through geological characteristics study and numerical simulation test, explores the influence of lithologic combination in Taiyuan Formation of Carboniferous System on coal mining.The feasibility of carbonatite on the top of Ordovician system as relative aquifuge is discussed, and the water-conducting height of Ordovician karst water in the 3# and 15# coal seams under different mining conditions is simulated.The results show that the mining of No.3 coal seam has limited influence on the Ordovician karst water, and the integrity aquifuge on the floor effectively prevents water inrush.However, during the mining of the 15# coal seam, because of the small spacing between the floor and the roof of the Ordovician system, the risk of a water inrush of the floor was posed by the combination of karst water and mine pressure.The effect of relative aquifuge can be fully utilized, but the defective aquifuge needs to be repaired.This study provides technical references for the safe mining of coal in Sima Coal Mine in the future.