In this work, the combustion, thermal, and mechanical performances of long-glass-fiber-reinforced polypropylene/intumescent flame retardant (LGFPP/IFR) composites with different contents of polyamide 6 (PA6) as a charring agent were investigated by limiting oxygen index (LOI), UL-94 test, cone calorimetry, thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), and mechanical property test. The results showed that LGFPP/IFR/PA6 composites exhibited much better flame retardancy due to the dense char layer structure, as proved by SEM. When 15 wt% PA6 was added, the LGFPP/IFR composites had the best flame retardancy, and LOI increased by 55.1 %, and the UL-94 reached V-0 rating. The cone calorimeter tests indicated that PA6 could prevent the transmission of heat and the volatilization of flammable substances, thereby reducing the heat release of “second burning”. According to TGA analysis, the carbon layer began to form at a lower temperature due to the synergistic effect between IFR and PA6. Moreover, the calculation of apparent active energy (E a) revealed that PA6 effectively increased the E a values of the composites, resulting in a better thermal stability and flame retardancy. In addition, PA6 enhanced the mechanical properties of the composites effectively.