Abstract: As open-pit mining in the west mining area of the Songshunangou Gold Mine gradually approaches its final boundary, the transition to underground mining has become an important approach for deep resource development. During the open-pit to underground transition, the boundary crown pillar acts as a key isolation structure connecting the pit bottom and the underground stopes, and its thickness directly affects slope stability, underground mining safety, and surface subsidence control. To determine a reasonable crown pillar thickness, this study systematically analyzed the engineering geological and hydrogeological conditions of the mining area and applied several theoretical methods, including the thickness-span ratio method, load transfer intersection line method, Protodyakonov arch theory, and the K.B. Rubenietha empirical formula, to calculate the safety thickness of the crown pillar under different stope span conditions. The calculated safe thickness obtained from these theoretical approaches mainly ranged from 14 m to 20 m. By considering unfavorable factors such as fractured rock mass structures, freeze-thaw cycles in cold regions, and groundwater effects, an engineering safety allowance was introduced based on the theoretical results, and the minimum safe thickness of the boundary crown pillar was finally determined to be 25 m. Furthermore, a three-dimensional numerical model coupling the open pit and underground stopes was established using Flac^3D to simulate the stress and displacement evolution during staged underground mining and backfilling. The simulation results show that under a crown pillar thickness of 25 m, the maximum compressive stress of the pillar is about 15 MPa, and the maximum displacement is 7.905 cm, both of which are within the rock mass strength and engineering stability limits, indicating that the crown pillar remains stable overall. The research results provide a scientific basis for the design of open-pit to underground mining at the mine.