It is important to research the existence of the top Ordovician water-resisting strata, under which the regional Ordovician karts aquifers pose great water pressure threat to the upper late Carboniferous coal seam mining. So we take the samples from JK-02 to do mercury test, SEM and microscopic identification. The results show: ① 0 ~10m of the top strata of Ordovician is made of crystal powder dolomite. Most of the pores of the strata are intercrystalline pores, intercrystalline dissolved pores, microspore as well as stylolite ,and part of which are filled by calcite and clay. Its porosity is 11.3242%; and permeability 6.2324;and diameter of volume median 66.3nm; capillary median pressure 6.88MPa; mercury withdrawal efficiency is 42.22%; maximum pore throat's diameter is 145.32μm, and effective pore volume is about 13% out of total pore volume. ②10 ~112m of the top Ordovician is mainly made of powder crystal limestone and calsilutites. The most of pores are micropore; its porosity is only 0.8409% and permeability 7.0438; and diameter of volume median 66.3nm; and capillary median pressure 5.497~34.391MPa; maximum pore throat's diameter is 0.0227～0.453μm; ③The number of pores and the porosity of the limestone are less than that of the dolomite. The limestone of the top Ordovician can serve as water-resisting layer for its lower permeability and compactness if there is no water-conducted faults.