Abstract: In order to improve the flame retardant of glass fiber (GF) reinforced polypropylene (PP) composites (GF/PP), by means of melamine cyanurate (MCA) molecular self-assembly in montmorillonite (MMT) suspension, a new synergistic charring agent MCA-MMT was prepared. FTIR, XRD, SEM and TGA were utilized to characterize the structure and thermal properties of MCA-MMT. Flame retardant composites MCA-MMT/(GF/PP) were prepared by melt blending of MCA-MMT, halogen-free intumescent flame retardant and GF/PP. The flame retardant efficiency and flame retardant mechanism of MCA-MMT toward GF/PP were investigated by limiting oxygen index (LOI) test, vertical burning test and cone calorimeter test. The mechanical properties of composites were also tested. The results indicate that the addition of MMT affects the hydrogen-bond interaction between cyanuric acid and melamine in the synthetic process of MCA, interrupts and inhibits the formation of large planar hydrogen bonding network, and reduces the molecular volume of MCA hydrogen bonding composite which makes the particles smaller. The UL-94 fire rating of MCA-MMT/(GF/PP) reaches V-0, the LOI is 31.3%. The flame retardant efficiency of MCA-MMT is higher than that of traditional MCA, it can reduce the heat release degree and total smoke released, and improves the flame retardant propertier of composites. The mechanism is the MMT with sheet-like structure can improve the char residue amount of MCA, makes the MCA-MMT/(GF/PP) after combustion forms the compact and dense char residue. The tensile and impact strength of MCA-MMT/(GF/PP) do not decrease comparing with those of MCA/(GF/PP).