During the transition of a mine from pillarless sublevel caving mining method to filling mining method,safe isolation layer of certain thickness need to be reserved.This paper used the simplified beam method to theoretically calculate the safe thickness of isolation layer in the goaf,which is 18.2 m.The Flac numerical simulation software was used to simulate the field model to verify the theoretical calculation results.The research finds that the tensile stress in the goaf is not changed much before and after filling,basically keeping below 0.3 MPa,while the surface stress remains basically the same.Research on the surface displacement shows that production safety is guaranteed when the thickness of safe isolation layer is 18.2 m.The numerical simulation results are consistent with the theoretical calculation results,verifying the reasonability of the value.