Abstract: Aiming at the resource utilization of industrial solid waste, this study systematically investigates the effect of partially replacing N220 carbon black (CB) with 600-mesh steel slag powder (600SS), calcium-based desulfurization ash (FGD), and its hydrothermal conversion product—calcium-based desulfurization ash whisker (CSW)—on the performance of styrene-butadiene rubber (SBR) composites. By analyzing the vulcanization characteristics, mechanical properties, microstructure, and thermal stability of the composites, the reinforcement mechanism and optimization pathway of the solid waste fillers were clarified. The results show that when 600SS and CSW are blended at a 1:1 ratio to replace 15% of CB, the composite exhibits optimal comprehensive performance: tensile strength reaches 17.41 MPa, tear strength is 90.53 kN/m, which is close to that of the pure CB system, and the safety during vulcanization processing is significantly improved. Microstructure analysis indicates that at low replacement ratios, the fillers are uniformly dispersed, and CSW enhances interfacial bonding through a "microfiber bridging" effect. However, when the CB replacement ratio exceeds 22.5%, filler agglomeration intensifies, leading to a decline in performance. Thermogravimetric analysis reveals that at a 15% replacement ratio, the thermal stability (Td ≈ 398°C) is comparable to that of the CB system, and the increased residue content provides enhanced flame retardancy. This study confirms that the blend of 600SS and CSW can effectively replace CB at an appropriate ratio, offering theoretical foundations and technical support for the green transformation of the rubber industry.