Abstract: Based on field engineering conditions, this study conducted a three-dimensional FLAC3D numerical simulation and established a whole-process simulation model, covering historical open-pit mining, underground excavation, open-pit backfilling, and long-term deep stoping, to explore the synergistic stability characteristics of open-pit backfilling and deep orebody mining in Tongqianshan. The evolution patterns of displacement, stress and plastic damage of the backfill, surrounding rock, horizontal isolation pillar and roadways at the 4 585 m level were systematically analyzed. The results show that during open-pit backfilling, the maximum compressive settlement of the backfill ranges from 6.0 to 7.0 cm, the additional settlement of the pit-bottom rock mass is −1.0 to −1.5 cm, and the cumulative additional settlement of the roadways is −0.9 to −1.2 cm, with basically no changes in stress and damage, indicating that backfilling exerts slight disturbance on underground workings. During underground mining, the deformation of the overlying rock, pillar, and backfill remains coordinated and controllable, the ore and rock masses exhibit reasonable stress redistribution, and no through-going failure occurs in key structures. Open-pit backfilling and deep mining exhibit good synergistic stability. The research findings can provide a theoretical basis and technical support for safe and efficient mining in mines transitioning from open pit to underground operation.