Abstract: On the basis of discussion on the predicting theory of water-inrush from mining floor, the authors put forward a main floor model. In the light of the first weighing span formula of main roof, the formation of the limit fracture span of main floor is discussed,and the relationship between the water inrush from mining floor and the characteristics of aquifuges and aquiefers as well as the mining boundary conditions is made clear. Finally,a converse prediction mdthod is proposed in which the past information of water-inrush is used for predicting whether the water-inrush happen or not in the similar op adjacent working faces.