Abstract: Caving-to-backfilling mining in underground mines is an important trend to maintain sustainable development and protect the ecological environment in the mining industry. In order to ensure the orderly connection of safe mining from caving method to back filling method in a large underground mine, the thickness and stability of the safety isolation layer of the mine are calculated and analyzed in depth. Firstly, the thickness of the isolation layer is calculated by means of the intersection method of load transfer lines, the stress condition of the beam and plate, and the platts theory, the thicknesses are 19.5 m, 20.1 m and 22.7 m; secondly, a three-dimensional refined ore body mining model is established, and three thickness schemes of 15 m, 20 m and 25 m of safety isolation layer in caving-to-backfilling mining are designed and compared. The finite difference calculation software FLAC^3D is used to simulate the whole process of stratified mining by caving method and the excavation of the first mining middle section by back filling method. The results show that when the thickness of the safety isolation layer is 20 m, the maximum subsidence displacement value is 21 cm, the minimum principal stress is 0.05 MPa, there is no tensile failure in the rock mass of the isolation layer, and the plastic failure area is scattered and the influence range is limited. Combining theoretical calculations and numerical simulation results, it is finally determined that the thickness of the mine safety isolation layer should not be less than 20 m. Leaving a safety isolation layer in the ore body can effectively reduce the mutual influence of the caving mining and back filling mining area, and ensure the safety of the deep ore body back filling mining, and the research method and results can provide a reference for the mines with similar conditions.