Abstract: Taking G3 hydrological hole in Tunlan coal mine as sampling aperture and according to comprehensive experiments such as the rock slice appraisal,X-Ray fluorescence spectrum analysis and rock microstructure analysis,lithologic characters of Fengfeng Group and pore structure are analyzed.Research illustrate that the upper Fengfeng Group is breccia limestone,while the lower is dolomicrite filled with coarser crystal gypsum in fracture without karst development.The pore's diameter≥10 μm in the upper Fengfeng group accounts for 27%,while that pore's diameter in the middle and lower occupies 11% and the diameter <0.1 μm of fine and supercapillary interstice takes up 59%,which means that the permeability of the lower Fengfeng group declines to a large extent.There is an aquifuge between the upper and lower Fengfeng group,in which the thickness of available water-resisting key strata is 35 m in the upper part and 70 m in the lower.