Abstract: The partial substitution of carbon black with steel slag powder (SSP) in the preparation of steel slag powder-carbon black/styrene-butadiene rubber composites (SSP–CB/SBR) is an effective approach to enhancing the high-value utilization of SSP. The compatibility of SSP–CB/SBR is fundamental to the development of multi-component composites. In this study, molecular dynamics simulations were used to construct a steel slag-SBR interfacial model and assess the interfacial compatibility of the two components. Further experimental analyses were conducted to analyze the effects of SSP fineness on the mechanical and flame-retardant properties of SSP–CB/SBR, addressing the limitations of molecular simulation. The results showed that the dicalcium silicate (C₂S) and tricalcium silicate (C₃S) fractions in steel slag effectively interacted with the SBR interface, exhibiting minimal temperature-energy fluctuations and reaching equilibrium. Radial distribution function and interaction energy calculations showed a pronounced molecular aggregation effect between C₂S, C₃S, and SBR, leading to a reduced spacing of 18 Å and enhanced binding strength. When the SSP particle size was controlled at 600 mesh, the tensile strength of SSPCB/SBR increased significantly to 16.91 MPa, representing a 17.19% improvement over the sample without SSP. The incorporation of SSP into the SBR system effectively improved the flame-retardant properties of SSPCB/SBR, as reflected in increased oxygen indices. This enhancement was attributed to the uniform distribution of SSP within the SBR matrix, which formed a thermal barrier during combustion. Further characterization using scanning electron microscopy and thermogravimetric analysis showed that SSP–CB/SBR prepared with 600-mesh had a uniform texture and a dense carbon layer after combustion. The presence of SSP effectively retarded the thermal decomposition of SSP–CB/SBR, resulting in improved combustion stability. These observations elucidate the flame-retardant mechanism of the composite materials, where SSP contributes to the formation of a stable carbon layer that inhibits heat transfer during combustion. In conclusion, this study demonstrates that SSP can serve as a viable partial replacement material for carbon black in SSP–CB/SBR, producing composites with enhanced mechanical and flame-retardant properties. The combination of molecular dynamics simulations and experimental analyses provides a comprehensive understanding of interfacial interactions and material properties, laying the foundation for the development of high-performance composites. 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.