In　order　to　explore　highly　efficient　flame-retardant　rigid　polyurethane　foam　(RPUF),　phosphorus/nitrogen　compounds　and　expandable　graphite　(EG)　were　successfully　incorporated　into　RPUF　by　a　free　one-spot　method.　The　combustion　results　showed　that　the　fire　safety　of　the　RPUF　samples　was　remarkably　improved　by　the　addition　of　phosphoric/nitrogen　compounds　and　EG.　With　the　incorporation　of　22.4　wt.%　phosphorus/nitrogen　compounds　and　3.2　wt.%　EG,　the　RPUF　composites　achieved　UL-94　V-0　rating.　Besides,　the　total　heat　release　and　total　smoke　release　of　RPUF　composites　were　reduced　by　29.6%　and　32.4%　respectively,　compared　to　those　of　the　pure　RPUF　sample.　PO•　and　PO2•　together　with　nonflammable　gaseous　products　were　evolved　from　phosphoric/nitrogen　compounds　in　the　gas　phase,　which　quenched　the　flammable　free　radicals　in　the　matrix　and　diluted　the　concentration　of　combustible　gaseous　products　generated　from　PRUF　during　combustion.　The　compact　char　residues　which　acted　as　excellent　physical　barriers　were　formed　by　catalysis　of　EG　and　phosphoric/nitrogen　compounds　in　the　condense　phase.　The　fire　hazard　of　RPUF　was　significantly　reduced　by　the　synergistic　effect　of　phosphorus-nitrogen　compounds　and　EG.　This　work　provides　a　promising　strategy　to　enhance　the　fire　safety　of　RPUF.　©　2020　by　the　authors.　Licensee　MDPI,　Basel,　Switzerland.