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[Display omitted] • An IFR containing acid, carbon, and gas sources was prepared by in-situ calcination. • HRR, EHC, THR, and TOC of fabric with IFR were less than those without IFR. • Carbonization process and fireproof mechanism of IFR on fabric were studied. A novel macromolecular carbon-nitrogen-phosphorous (C-N-P) intumescent flame retardant (IFR) containing acid, carbon, and gas sources was prepared by calcining polyphosphate (APP), pentaerythritol (PER), and melamine (MEL) through the direct heating method. The carbonization and expansion behaviors of IFR were investigated by testing the products of IFR, using nuclear magnetic resonance, X-ray photoelectron spectroscopy, transmission electron microscope, and scanning electron microscopy, at 360 °C, 450 °C, and 600 °C. Conjugated double bonds polymerized the nitrogenous polyaromatic structures as polyphosphate decomposed into phosphoric acid and diphosphate. Noncombustible gases released by MEL increased the internal pressure of the viscoelastic materials to make it expand. At temperatures below 450 °C, MEL promotes the formation of nitrogenous polyaromatic structures in the material, thereby optimizing the IFR char layer structure. The heat release rate, effective heat combustion, total heat release, and total oxygen consume of the IFR coated fabric were less than those of an uncoated fabric by 18%, 5%, 16%, and 13%, respectively, reducing the risk of fire hazard. The foam char formed during the burning of an IFR coated fabric controls the fabric's oxidation process and prevents its shrinking. [ABSTRACT FROM AUTHOR]