Abstract: Coal gasification slag (CGS) is a solid residue formed from inorganic minerals and residual carbonaceous particles during the coal gasification process. Its comprehensive utilization rate is low, and landfilling at a cost of 30−60 CNY/t is currently the primary disposal method. Therefore, the comprehensive utilization of CGS has received considerable attention. CGS is used as the raw material, and coal gasification slag powder (CGSP) is obtained after grinding. CGSP is then used to partially replace carbon black (CB) for the preparation of coal gasification slag powder–carbon black/styrene-butadiene rubber composites (CGSP-CB/SBR). The mechanical and flame-retardant properties of the CGSP-CB/SBR composites are systematically investigated. The chemical composition, mineral composition, particle-size distribution, specific surface area and pore structure, microstructure, structural composition, and thermal stability are characterized using X-ray fluorescence spectroscopy, X-ray diffraction (XRD), laser particle size analysis (LASP), an automatic specific surface area and pore structure analyzer, scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric-differential thermal analysis (TG-DTG), respectively. The results show that the vulcanization reaction is promoted and the vulcanization time is shortened by replacing N660 carbon black with CGSP. When 15% of CB is replaced by CGSP, the shortest vulcanization time and the highest cure rate index are obtained for the CGSP-CB/SBR composite. The minimum torque (M_L) values of all CGSP-CB/SBR composites are lower than that of the CB/SBR composite (0.318 N·m), and the M_L value gradually decreases with increasing CGSP replacement of CB. The lowest M_L value of 0.259 N·m is obtained for CGSP-CB/SBR-III. The flowability of the CGSP-CB/SBR composites is improved, while the Mooney viscosity is reduced and the plasticity is increased by the incorporation of CGSP. When 10% of CB is replaced by CGSP, a tensile strength of 18.15 MPa and a tear strength of 96.65 kN/m are obtained for CGSP-CB/SBR-I, representing a 12.51% increase in tear strength compared with the CB/SBR composite. When 10%−20% of CB is replaced by CGSP, the residual mass fractions of the CGSP-CB/SBR composites are gradually increased to 6.14%, 6.50%, and 9.38%, respectively, while the limiting oxygen index values are increased to 19.3%, 19.8%, and 19.4%, respectively. Consequently, improved thermal stability and flame-retardant performance are achieved.