Coal mining causes the movement and deformation of the overburden strata, which leads to the buildings collapse and the ecological environment destruction. To study the development law and stability of mining-induced separation layer in inclined strata of Huainan mining area, based on the key strata theory, an engineering geomechanical model of mining-induced overburden grouting consisting of main critical strata, grouting barrier layer and sub-critical strata was proposed, and the mechanical criterion of stability of grouting separation layer was established. Further, using discrete element numerical simulation, horizontal and inclined strata models were respectively built to simulate the evolution law of fractures and the development of grouting separation beds during coal seam mining. The results show that the grouting barrier layer is not penetrated through the water-conducting fractures in both horizontal and inclined strata. The grouting separation layer is developed at the bottom of the main key strata, and the opening of grouting separation layer in horizontal strata is larger than that in inclined strata. The numerical simulation proves that the engineering geomechanical model of mining-induced overburden grouting can effectively predict the reliability of developed separated strata.