Abstract：In order to explore the safety mining of Carboniferous-Permian coal resources，especially a great deal of lower coal seams being threatened by Ordovician karst water，relative impermeability of Ordovician top is demonstrated through studying lithology，filling degree of the karst fissures as well as water leakage of drilling fluid and aquifer shown in well logging based on the viewpoint that water-resisting strata exist in Ordovician top and can be considered as water-resisting key strata. Furthermore，the main influencing factors on impermeability of coal seam floor are analyzed；and divisional risk assessment of floor water-inrush from Ordovician karst is performed. Overall，accuracy of risk assessment on floor water-inrush can be improved by considering the relative water-resisting strata in Ordovician top as part of the whole ones.
