The performances of a novel char forming agent containing spiro and caged bicyclic phosphate (SBCPO) and its mixture with ammonium polyphosphate (APP) were investigated. The flame retardancy of the polypropylene (PP)/SBCPO/APP system was evaluated by the limiting oxygen index (LOI), the UL-94 test, and the cone calorimeter test. The maximum LOI value of PP/intumescent flame retardant (IFR) could reach 30.5 and passed the UL-94 V-0 rating. An obvious synergistic effect could be observed between SBCPO and APP by TGA. DSC, and synergistic effectivity studies, which remarkably improved the flame retardant properties of PP. FTIR analysis indicated that APP could accelerate the decomposition of SBCPO due to the interaction between them. The thermal degradation kinetics results of the PP/IFR system by the Kissinger method and the Flynn-Wall-Ozawa method showed that the apparent activation energy (E-a) increased with the increase of content for flame retardants.