Abstract: The artical uses FEM software ANSYS to simulate the mining process of under-protecting coal stratum.Through the changing situation of strain and stress fields of protected seam when advancing different distances,the relief range of protective layer is determined.The simulation result shows that the swelling deformation rate of the protective layer is 0.35% to 1.18%,which basic agrees with the measured swelling deformation rate of 0.56%.The stable and effective relief zone is located between 50 m ahead of open-off cut and 60 m behind the working face,which is identical with the actual measurement data.