Abstract: In response to the lack of synergy between structural parameters and backfill strength in downward drift filling mining methods applied to complex ore bodies, a collaborative optimization study was conducted based on the Maoping lead-zinc mine to provide a foundation for safe mine expansion. A three-dimensional numerical model was built by employing Flac^3D software to simulate the mining processes of three drift sizes, including 3.5 m×3.0 m, 4.5 m×4.0 m, and 5.5 m×5.0 m. By adopting a bisection-based strength search method, the critical strength requirements of the bottom-layer backfill for each drift size were determined. Results indicate that under the drift size of 3.5 m×3.0 m, the critical cohesion is 350 kPa, corresponding to the uniaxial compressive strength of 0.96 MPa. When the drift size increases to 4.5 m×4.0 m, the critical cohesion rises to 410 kPa, with the strength requirement of 1.13 MPa. When the drift size grows to 5.5 m×5.0 m, the critical cohesion reaches 620 kPa, with the strength requirement of 1.71 MPa. By integrating the measured average backfill strength of 4.41 MPa and safety factor of 3.6, the actual required strength for the three drift sizes was calculated as 3.46 MPa, 4.07 MPa, and 6.16 MPa, respectively. Analysis reveals that in the current backfill strength conditions, expanding the drift size from 3.5 m×3.0 m to 4.5 m×4.0 m is feasible, whereas further expansion to 5.5 m×5.0 m results in insufficient strength. This study elucidates the quantitative relationship between drift size expansion and backfill strength, thus providing concrete guidance for optimizing drift parameters and backfill mixture design in similar conditions, and holding guiding significance for safe and efficient mining.