A novel hyperbranched charring agent containing s-triazine, diphenylmethane and urea groups (HBPU) was synthesized from melamine and 4,4'-diphenylmethane diisocyanate, which is hydrophobic and thermally stable at high temperature (residual weight being 40% at 800 degrees C). HBPU and ammonium polyphosphate (APP) are synergistic for the improvement of flame retardancy and smoke suppression of polypropylene (PP). With the addition of 30% of APP and HBPU (2:1 or 3:1), PP composites could reach V-0 rating at UL-94 test, and their peak heat release rates (PHRR) were reduced by 78.6% and 81.3%, respectively. HBPU also improved the compatibility of APP with the PP matrix with a result of enhanced mechanical properties. In this intumescent flame retardant system, HBPU served as carbonization and blowing agents simultaneously. In addition, the interaction of HBPU and APP as well as their mechanism of flame retardation were deeply discussed.