Abstract: To improve the flame retardancy and ceramization efficiency of ceramifiable silicone rubber sealant (CSR), polyborosiloxane (PBS) was synthesized using α,ω-dihydroxy polydimethylsiloxane (PDMS) and boric acid as raw materials. CSR/Pt/PBS composites were prepared via the blending method. The synergistic effect of PBS and Karstedt's platinum compound (Pt) on the mechanical properties, thermal stability, flame retardancy, and ceramization performance of the composites was systematically investigated. Results indicated that the synergistic effect of the two endows the CSR/Pt3/PBS10 composite system with the optimal flame retardancy: the limiting oxygen index (LOI) is increased from 33.6% to 39.3%, the vertical burning rating reaches UL94 V-0, and the peak heat release rate is reduced by 66.1% compared with pure CSR. After high-temperature ablation, the system forms a dense and continuous ceramic layer. The three-point bending strengths at 800 ℃ and 1200 ℃ are 12.94 MPa and 40.07 MPa, respectively, and the average backside temperature during the fire resistance test is as low as 90.1 ℃, demonstrating excellent structural integrity and fire insulation performance.