Acrylonitrile-butadiene-styrene terpolymer (ABS) resin is currently a high-performance synthetic resin widely used in industries such as automobiles and electronics, with a huge demand. However, its poor flame retardancy limits its application in products with high fire resistance requirements. In order to improve the flame retardancy of ABS resin, preliminary experiments showed that phosphoric acid flame retardants [ammonium polyphosphate (APP), triphenyl phosphate (TPP), and isopropylated triphenyl phosphate (IPPP)] can enhance the flame retardancy of ABS, and form a novel ABS composite with benzoxazine and zirconium hydrogen phosphate. Through testing the TG-FTIR, SEM, and HRR data of the composites, the following conclusion was drawn: Phosphoric acid flame retardants can pyrolyze to form phosphoric acid and pyrophosphate with strong dehydration effect, promote dehydration and carbonization of the ABS benzoxazine composite system, forming a carbon layer to improve flame retardancy; Bisphenol A type benzoxazine exhibits significantly higher flame retardancy than aniline type benzoxazine in the composite system; The introduction of zirconium hydrogen phosphate as a synergistic carbonizing agent, although effectively improving the residual carbon rate, may have a certain impact on the density of the carbon layer due to its layered structure.