Abstract: Reasonable thickness of the boundary pillar is a key factor for the stability of underground mining transferring from open-pit.Using the simulation software FLAC3D, ten goaf models with different thicknesses under the same span were established to analyze the displacements, stresses and plastic zones of key monitoring points combined with the traditional evaluation index such as safety factor.Finally, the results were compared with four traditional roof prediction methods.The research indicated that the displacement value of roof was negatively correlated with the thickness, and the safety factor was nonlinear.According to the mutation point of displacement curve, the reasonable thickness of the boundary pillar was 21 m.Relative error of numerical simulation method was only 2.2%, which was far less than those of the traditional methods.The predictive value of the model is of high reference value, and could guarantee the high security and resource utilization.