The focus of prevention and control of water inrush from floor is to study the evolution of the intact floor from aquifuge to water-conducting. The coal seam mining causes the floor rock to bear the cyclic loading of compression – tension –compression , and causes the elastic modulus change. In this paper, the damage variable is elastic modulus, using double scalar D-P elastoplastic damage constitutive model, we established numerical model according to the conditions of Chengzhuang Coal Mine , and analyzed the evolution law of water-conducting of mining floor. The main conclusions are as follows: (1) Floor damage depth increases with the increase of the roof hanging area, and leads to the fracture depth increases again in last calculation step. Under the influence of filling body, the growth rate of the floor compression damage depth at the position of the coal wall is rapidly decreasing (the first roof caving), and finally reaching stability (the periodic caving). (2) Compression and tensile damage zones exist in mining floor simultaneously. And these zones are connected mutually, which determines the location of the water channel. (3) The elastic modulus of the filling body has a great influence on the failure depth of the floor; and the low value will lead to the rapid increase of the failure depth. The results of water injection experiment are basically consistent with the results of numerical simulation.