<p>In basic oxygen furnace steelmaking, high-velocity top-blown oxygen jets induce intense splashing of slag and molten steel, a phenomenon that directly governs both smelting reaction efficiency and the service life of the converter lining. Slag splashing protection is therefore widely recognized as the core technology for extending converter campaign life. However, existing numerical studies struggle to strike a balance between high-fidelity capture of the gas–slag–molten steel interface and efficient tracking of discrete droplets, leaving the coupling mechanism between splashing behavior and the formation of the wall protective liquid film still unclearly understood. To solve this problem, this study develops a fully coupled 3D numerical framework for gas–slag–molten steel flow. It integrates bidirectional VOF-DPM conversion, adaptive mesh refinement and the Eulerian wall film model. The model is validated against classical experiments, with a prediction deviation of less than 3% for splashed mass. Results show that top-blowing flow rate is the dominant factor controlling splashing intensity. When the flow rate increases from 6.35 to 11.76 Nm³/h, droplet generation rate rises by 650% and wall film area expands by 320%. The effect of oxygen lance height strongly depends on the flow rate. At low flow rates, 200 mm lance height gives the strongest splashing. At medium and high flow rates, 220 mm lance height optimizes droplet residence time to 0.5 to 0.6 seconds. High flow rates also narrow slag droplet size distribution to 2 to 4 mm, which improves uniform slag deposition on the furnace wall. These findings clarify the multiscale evolution of splashing and provide quantitative guidance for industrial slag splashing optimization.</p>