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In this paper, a melamine formaldehyde resin modified with tris(2-hydroxyethyl) isocyanurate (THEIC), named MF-THEIC, was synthesized via a solid-phase reaction between hexahydroxymethyl melamine (HMM) and THEIC. MF-THEIC was firstly used as a charring agent in an intumescent flame-retardant system together with APP as well as 4A-zeolite in flame-retarding PP. APP/MF-THEIC (3/1) can effectively reinforce the flame retardancy of PP at a total 24.0 % additive amount, moreover, 1.5 % 4A-zeolite is sufficient to achieve an optimum synergistic effect. The corresponding composites (Sample-17) had passed UL-94 V-0 classification well. The synergistic flame-retarding mechanism was systematically investigated by SEM, 31P NMR, TGA, FTIR spectra, and TG-MS. 4A-zeolite can react with APP to generate highly-active phosphates with good catalytic efficacy on polyphosphate esterification, and then to promote the formation of the char layer, simultaneously, 4A-zeolite is helpful to enhance the char graphitization degree and the strength of the intumescing char layer, thereby reinforcing its flame retardancy via a synergistic effect. • A novel charring agent MF-THEIC was synthesized via a solid phase reaction between HMM and THEIC. • APP/MF-THEIC promoted the charring due to the chemical reactions. • 1.5 % 4A-zeolite synergistically improved the flame retardancy with APP/MF-THEIC in PP. • 4A-zeolite increased the graphitization degree and the strength of the intumescing char layer. [ABSTRACT FROM AUTHOR]