Abstract: Polycarbonate/acrylonitrile styrene acrylate (PC/ASA) composites modified with two kinds of synergistic flame-retardant systems, p-bis-(diphenylphosphoryloxy)-benzol (BDPB)-silsesquioxanes, phenyl, hydroxy-terminated(SPH) and BDPB-organically modified montmorillonite(OMMT), respectively, were prepared by melt compounding. The microstructure、combustion behavior and dynamic mechanical properties were investigated by XRD, TG, limiting oxygen index(LOI), SEM, XPS and DMA test. The results indicate that the BDPB-SPH and BDPB-OMMT flame-retardant systems exhibit a synergistic effect. The BDPB-SPH-PC/ASA and BDPB-OMMT-PC/ASA composites could reach V-0 rating in vertical burning test (UL-94), and the LOI of the composites reach 30.4% and 31.2%, respectively. The residual of BDPB-SPH-PC/ASA and BDPB-OMMT-PC/ASA could increase to 12.43% and 14.24%, respectively. With the loading of BDPB-SPH and BDPB-OMMT flame-retardant systems, the microstructure of residual layer becomes more compact. BDPB, SPH and OMMT tend to accumulate at the surface during combustion, which could promote the formation of stable layer and as a result enhance the flame retardant properties. The glass transition temperature(T_g) of BDPB-SPH-PC/ASA and BDPB-OMMT-PC/ASA composites shift to low temperature, and the storage modulus of the composites are enhanced before T_g.