The effects of allophane (ALL) as a synergistic agent on the flame retardancy and thermal stability of intumescent flame-retardant (IFR) polypropylene composites were studied by limiting oxygen index (LOI), UL-94 test, cone calorimeter test (CONE) and thermogravimetric analysis. The incorporation of ALL led to enhanced thermal stability, LOI value and UL-94 ratings. CONE tests indicated that heat release rate, peak rate of heat release, smoke production rate, total smoke production and mass loss values of PP/IFR/ALL sample were much lower than those of PP/IFR and pure PP samples. Fourier transform infrared spectrometry result proved the presence of silicoaluminophosphate which can act as a catalytic agent to enhance the oxidative dehydration crosslinking charring process. Scanning electron microscopy and Raman spectra observation demonstrated that ALL could promote forming homogenous and compact char layer. The mechanical properties of PP composites were also improved by the loading of ALL.