Abstract: A novel intumescent flame retardant, hyperbranched polyphosphoramide (HBPPA), was synthesized by using phosphorus oxychloride and diaminodiphenylmethane as raw materials and triethylamine both as a catalyst and as an acid-binding agent, and its chemical structure and thermal stability were characterized. A series of flame?retardant polypropylene (PP)-based composites were prepared by melt blending with HBPPA and ammonium polyphosphate (APP), and their flame-retardant performance, thermal stability, char morphology and pyrolysis properties were analyzed. The results indicated the composite containing 30 wt% HBPPA and APP at the mass ratio of 2∶1 presented the highest limiting oxygen index of 28.2 % and achieved a UL 94 V-0 classification in the vertical burning test. Such good flame-retardant performance may ascribed to a synergistic effect between HBPPA and APP, which facilitates to form a compact and continuous char layer to inhibit the impact of degraded gaseous and also to act as a physical barrier to hinder the exchange of external oxygen and internal flammable volatile gases and the heat transmission.