Abstract: The flame-retardantcompounds based on polycarbonate (PC)/acrylonitrile-butadiene-styrene copolymer (ABS) alloys and hexaphenoxycyclotriphosphazene (HPCTP) were prepared by a melt extrusion method, and the effects of polyethylene grafted maleic anhydride (PE-g-MAH) and HPCTP on the thermal stability, flame-retardant performance and mechanical properties of PC/ABS alloys were investigated by thermogravimetric analysis, limited oxygen index (LOI), vertical burning experiment, cone calorimeter test, electronic universal testing machine and impact testing machine. Furthermore, the morphology of residual char was analyzed by scanning electron microscopy after combustion of PC/ABS alloys, and the relevant flame-retardant mechanisms were discussed. The results indicated that the flame retardant PC/ABS alloys achieved the optimum comprehensive properties by using PE-g-MAH as a compatibilizer when the mass ratio of PC/ABS was set to 7/3 and the content of HPCTP set as 15 wt %. In this case, the PC/ABS alloy obtained an LOI of 26.4 vol %. Furthermore, the PC/ABS alloys achieved the minimum values in heat release rate and total heat release as well as a V-0 classification in the vertical burning experiment. The alloys also gained the tensile strength of 55 MPa and the notched impact strength of 32.9 kJ/m2.