In the presence of Karastedt catalyst, a linear polymer was synthesized by the hydrosilylation with double-decker-shaped silsesquioxane(DDSQ)and propargyl-derived ODOPN, yield 88.5%. The copolymer was characterized by ∼1H NMR, ∼(31)P NMR, ∼(29)Si NMR, and TGA. The material showed good thermal stability. When 10 wt% polymer was added to PC/ABS alloy, the flame retardant efficiency was greatly improved. The LOI value increased from 23.0%(pure PC/ABS)to 27.2%, and the UL-94 achieved V0 level. The phosphorus-silicon flame retardant showed a synergistic flame retardant effect.