Abstract: Focusing on the problem of reasonable thickness that the boundary pillars should be reserved during transferring open-pit into underground mining,and basing on the analysis of the detailed geological information,mining information and the reality situation,the mechanical modeling for the sijiaying Iron Mine is established by using FLAC^3D numerical simulation software.The stress distribution and the plasticity zone distribution of the boundary pillar are simulated and analyzed with the model.And how different thickness of the boundary pillars and top pillars and various distance between pillars affect the stress distribution and the plastic zone distribution of the boundary pillars are studied,so the reasonable thickness and the distance of the boundary pillars or other relative parameters are determined,all these provided scientific direction for producing with great efficiency and safety,when it is on time for transferring open-pit into underground smoothly.