A novel trisilanol polyhedral oligomeric silsesquioxane-containing phosphorus (DPCP-TPOSS) was synthesized from trisilanolphenyl polyhedral oligomeric silsesquioxane (TPOSS) and diphenyl chlorophosphate. DPCP-TPOSS was characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), hydrogen nuclear magnetic resonance ( 1 H-NMR), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Then a novel type of room-temperature vulcanized silicone rubber (RTV)/DPCP-TPOSS composite was prepared. Properties such as swelling behavior, tensile strength, elongation at break, thermal stability and flame retardance were researched and compared. Results showed that RTV/DPCP-TPOSS-3 and RTV/DPCP-TPOSS-5 composites exhibited the best tensile strength and elongation at break, 4.5 MPa and 427%, which was 25% and 32% higher than that of pure RTV. TGA tests demonstrated that RTV/DPCP-TPOSS-3 owned the highest char residues, 39.7%. T max of RTV composites was increased from 531.9°C to 557°C with the incorporation of DPCP-TPOSS. Moreover, the addition of DPCP-TPOSS led to considerably increase of the fire-retardant performance.