Abstract: The main target coal seam of Renlou Mine is directly covered with a 5~15m thick gravel aquifer, which seriously threatens the safety of mine, so that 80m vertically high safety pillar under water-bodies are designed to retain. To demonstrate the safety and reliablity of the pillar, the tests for hydrogeological conditions of mining area, rock mechanics, water-rock physical properties are conducted. In result, the gravel layer is of higher volume of cementing clay materials, weak ability of vertical permeation; the mudstone near basement of work face belongs to loose-medium rock type; the content of clay minerals in weathered zone of basement is relatively high, resulting strong regenerating watertightness, due to volume dilatation afeter water absorbing, which is of double functions of watertight and restrainting continuous development of producing fractures under mining impact conditions. The analogy simulation, computer simulation and formula calculation are conducted, too. Based on above stared results, the safety pillar is thinned to 65m. The withdrawal was finished in May, 1998. The sum of produced coal is up to 4.6× 10⁴t.