This Montmorillonite modified by melamine (MA-MMT) was prepared via cation exchange reaction by using melamine salt as intercalation reagent. MA-MMT and Na-MMT was combined with intumescent flame retardant (IFR) to be adopted into polypropylene (PP), respectively. The synergistic effect between MA-MMT and IFR and the influence of melamine in MMT layers on fire-resistant performance was evaluated. Results of limited oxygen index (LOI) tests and UL-94 tests indicate that melamine salts in MMT layers behaved better than Na-MMT in PP/IFR system. According to the results of cone calorimeter tests and scanning electron microscope (SEM), it concludes that melamine salts act as gas agent to provide migration impetus and expanded power, which caused a well-structured and strong char that had better ability to endure heat erosion. A good synergistic effect between MA-MMT and IFR is constructed.