Abstract: The use of steel slag as a partial substitute for carbon black in the preparation of steel slag-rubber composites represents an effective way to enhance the high value-added utilization of steel slag. The compatibility between steel slag and rubber is a key factor in the performance of composites. In this study, the interfacial model between steel slag and rubber was established using molecular dynamics simulation to verify the interfacial compatibility. In addition, experimental studies were conducted to analyze the effects of different finenesses of steel slag on the mechanical properties and flame retardancy of steel slag-rubber composites, thus compensating for the limitations of molecular simulations. The molecular dynamics simulations showed that the main components of steel slag, C?S (dicalcium silicate) and C?S (tricalcium silicate), have good compatibility with the rubber interface. The temperature-energy fluctuations were small and stabilized, indicating that the system reached equilibrium. Radial distribution function (RDF) calculations showed that when C?S and C?S were blended with rubber, there was a significant molecular aggregation effect, and the ionic spacing was narrowed down to 18 ?. This aggregation effect suggests that there are strong intermolecular interactions at the steel slag-rubber interface, which is crucial for the overall performance of the composites. The experimental results further confirmed the molecular simulation findings. When the particle size of steel slag was controlled at 600 mesh, the tensile strength of the steel slag-rubber composite was significantly increased to 16.91 MPa, which was 17.19% higher than that of the sample without steel slag. Incorporation of steel slag into the rubber matrix also significantly enhanced the flame retardancy of the composites with an increase in the oxygen index (LOI). The enhancement in flame retardancy was attributed to the uniform distribution of steel slag in the rubber matrix, which was able to form a thermal barrier during the combustion process. Further characterization of the composites by scanning electron microscopy (SEM) and thermogravimetric analysis (TGA-DTG) showed that the steel slag-rubber composites prepared using 600 mesh steel slag were homogeneous in texture, with a consistent carbon layer after combustion. The presence of steel slag effectively retarded the thermal decomposition process of the composites, thereby improving the uniformity of combustion. This observation reveals the flame retardant mechanism of the composites, i.e., the steel slag particles contribute to the formation of a stable carbon layer, which inhibits heat and mass transfer during the combustion process. In summary, this study shows that steel slag can be effectively used as a partial replacement material for carbon black in rubber composites to prepare materials with improved mechanical and flame retardant properties. The combination of molecular dynamics simulations and experimental analyses provides a comprehensive perspective for an in-depth understanding of interfacial interactions and material properties, and lays the foundation for the development of advanced steel slag-rubber composites with tunable properties. This study not only demonstrates the potential of steel slag as a sustainable and cost-effective filler material, but also advances the field of high-performance composites by introducing novel design and optimization concepts.