Reconstituted tobacco sheet (RTS) was first modified by ternary intumescent flame retardant (IFR) coating, composed of chitosan (CS), ammonium polyphosphate (APP), and montmorillonite (MMT) by basepaper coating techniques. The effects of composite coating on the combustion behavior and thermal degradation of RTS were studied by micro-scale combustion calorimetry (MCC) and TG-FTIR. MCC experiments confirmed that the CS-APP-MMT-RTS exhibited a 56.1% reduction in total heat release and a 42.8% reduction in peak heat release rate, relative to the uncoated RTS. TG-FTIR results showed that CS-APP-MMT coating reduced the maximum mass loss rate and the release of harmful gaseous products at the stage of char decomposition. Moreover, for CS-APP-MMT-RTS cigarette, the highest temperature of burning cone decreased by 39.0 ℃, thus demonstrating the high reduction of temperature and flammability.