Abstract: Aiming at the problem that the stability of the stope is difficult to guarantee after the open�pit to underground mining of the II~III line +120~+180 m ore body in a calcite mine，in order to determine the reasonable structural parameters of the stope and avoid the safety risk of mining，the theoretical calcu‐ lation and numerical simulation are combined to optimize the two key structural parameters of the top pillar thickness (4，5，6 m) and the width of the pillar (4，5 m). The stability of the stope under different parame‐ ters is systematically analyzed from three dimensions of stress distribution，displacement deformation and plastic zone distribution. The results show that the stope can remain basically stable under the three kinds of roof pillar thickness. The maximum Z-direction displacement of the roof after mining is 2.41，2.37 and 2.31 mm，respectively，which are far less than the safety control threshold of 30 mm. The maximum princi‐ pal stresses in the middle of the pillar are 5.94，5.76 and 5.51 MPa，respectively，which are less than the uniaxial compressive strength of 24.49 MPa of the ore body，and will not cause large-scale damage to the pillar and roof，which can ensure the safety of the lower section mining. Under the thickness of 4 m top col‐ umn，the filling body has local penetrating failure，and the pressure bearing capacity is greatly reduced， which has potential safety hazards. The stope stability under the two pillar widths meets the engineering re‐ quirements. The maximum principal stress in the middle of the pillar is 5.76 MPa and 6.35 MPa respective‐ly，and the maximum displacement in the Z-direction of the roof is 2.37 mm and 1.82 mm respectively. There is no penetrating plastic failure in the pillar，which can ensure the safety of mining. Considering the safety production，ore recovery rate，production capacity and engineering economy，it is suggested that the thickness of the top pillar of the stope should be selected as 5 m，and the width of the middle pillar should be selected as 4 m，which provides technical support for the underground mining engineering prac‐ tice of the mine.

Key words: open-pit to underground mine, stope parameter optimization, numerical simulation, stope stability