Abstract: According to the geological condition, floor failure depth of lower coal group was carried out a theoretical calculation, and on this basis, floor water irruption condition was analyzed by water inrush coefficient method.The paper simulated destruction law and crack evolution law of the floor under the combined effects of thin seam group mining dynamic stress and water pressure by using discrete element numerical simulation software UDEC, and obtained that the floor failure depth would reach 41 m. It was likely to happen floor water irruption, and the water prevention work of floor should be strengthened.