Two kinds phosphorus flame retardants, resorcinol bis (diphenyl phosphate) (RDP) (mainly condensed phase flame retardant) and triphenyl phosphate (TPP) (mainly gaseous phase flame retardant), were selected to prepare flame retardant polycarbonate (PC) /acrylonitrile-butadiene-styrene copolymer (ABS) alloy by melt blending.The effects of RDP/TPP on the flame retardancy of the alloy were investigated by vertical combustion test and cone calorimetry. The micro morphology of the carbon residue was observed by scanning electron microscope (SEM). The toughening and compatibilizing effect of methyl methacrylate-butadiene-styrene (MBS) on the flame retardant alloy was studied by tensile and impact tests, and the effects of MBS on the phase interface and the morphology of the alloy were observed by SEM. Results show that the condensed phase flame retardant of RDP and the gas phase flame retardant of TPP have synergistic effect, which can promote the formation of phosphate structure in PC/ABS alloy, thus help the system to form a continuous and compact carbon layer.