Interval layers are a key to ensure the safety of the transition from open pit to underground mining. Through simulation, finite element software is used to get the safety thickness of interval layers for Gushan Iron Mining. Firstly, a numerical simulation is carried out by using the software of ansys on several regular goaf models which have the same span(27.5m), but different thicknesses of ore pillar and different heights of goaf, optimally selecting the goaf height as 16 meters. Then, numerical simulation are created on other goaf models with different spans and thicknesses of interval layers and safety thickness of interval layers are obtained for these cases. Then five typical theoretical calculation methods are chosen to calculate its safety thickness. The result is similar with the former one. At the last, by making a combination of these two results, a consequence for different spans is got. And it is recommend 18 meters as safety thickness of interval layers for Gushan Iron Mining.