Developing flexible polyurethane foam (FPUF) with good anti‐ignition and self‐extinguishing performance is the fundamental way to prevent fire hazards in application. A binary composite system containing ammonium hypophosphite (NHP) and expandable graphite (EG) was applied to prepare flame‐retardant FPUF. The co‐action of NHP and EG exhibited high‐efficiency synergistic effect in improving the anti‐ignition, self‐extinguishing, and carbonization performance of FPUF. The analysis of micromorphology, element retention and chemical structure character on charring residue, reveals the physical–chemical synergistic carbonization mechanism of NHP/EG composite. The real‐time tracing on thermal‐decomposition volatiles, discloses the behaviors of NHP/EG composite in suppressing volatile fuel supply and toxic fume production. With an addition of 11 wt.% NHP and 7 wt.% EG, 11NHP/7EG achieved a limited oxygen index value of 30.3%, passed horizontal burning class HF‐1, and retained 55.6 wt.% charring residue. The peak of heat release rate and total heat release of 11NHP/7EG were both reduced by more than 50%, compared with that of pristine FPUF. The synergistic flame‐retardant mechanism of NHP/EG composite provides a valuable reference for high‐performance flame‐retardant FPUF manufacturing.