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Glass fiber reinforced polypropylene (GF-PP) composites have high flammability on account of wick effect which leads to accelerated flow of the polymer melt along the glass fibers (GF) surface to the flame zone. In this study, dipentaerythritol (DPER), a charring agent, was adsorbed on the GF surface through the hydrogen bond between silane coupling agent and DPER. DPER has a synergistic effect with the intumescent flame retardants (IFR) added in the composites, which can induce interfacial carbonization on the surface of GF, thus transforming the intrinsic smooth GF surface into roughness one. In this way, the negative effect of the wick effect in flame retardancy is weakened. Moreover, the char residues remained on the surface of GF can bring an improved adhesion between GF and char residues formed in the resin so that a more stable barrier char layer is formed. The PP composites with 20 wt％modified glass fiber (M-GF) and 30 wt％IFR can achieve the UL-94V-0, and its limiting oxygen index (LOI) value increased from 16.5％to 29.5％. Simultaneously, the heat release rate (HRR), total heat release (THR) and total smoke release (TSR) decreased significantly, and the peak of heat release rate (PHRR) reduced 60.6％compared with GF-PP.