Abstract: The flame retardant polycarbonate (PC)-acrylonitrile-butadiene-styrene (ABS) blends was prepared by melt blending with resorcinol bis(diphenylphosphate) (RDP) (mainly based on condensed phase flame retardant mechanism) and nano SiO₂ as the synergist. The effects of nano SiO₂ and RDP on the flame retardancy and combustion behavior of PC-ABS blends were investigated by vertical combustion (UL94) and cone calorimetry (Cone). SEM was used to observe the micro morphology of burning carbon residue, and EDS was used to analyze the change of element content on the surface of carbon layer, so as to further explore the synergistic flame retardant mechanism of nano SiO₂ and RDP in the condensed phase of PC-ABS. The effects of nano SiO₂ and RDP on the mechanical properties of PC-ABS blends and the toughening and compatibilizing effects of methyl methacrylate-butadiene-styrene (MBS) on the blends were studied by tensile and impact tests. The results show that nano SiO₂ and RDP can form Si—O—P compound in the condensed phase, which can enhance the combustion carbon layer, thus improving the flame retardancy of PC-ABS blends; The impact strength and elongation at break of the PC-ABS blends can be increased with the addition of MBS, but the flame retardancy can be decreased.