A　novel　flame-retardant　synergist,　chitosan/urea　compound　based　phosphonic　acid　melamine　salt　(HUMCS),　was　synthesized　and　characterized　by　Fourier　transform　infrared　spectroscopy　and　31　P-NMR.　Subsequently,　HUMCS　was　added　to　a　fire-retardant　polypropylene　(PP)　compound　containing　an　intumescent　flame-retardant　(IFR)　system　to　improve　its　flame-retardant　properties.　The　PP/IFR/HUMCS　composites　were　characterized　by　limiting　oxygen　index　(LOI)　tests,　vertical　burning　tests　(UL-94　tests),　microscale　combustion　calorimetry　tests,　and　thermogravimetric　analysis　to　study　the　combustion　behavior　and　thermal　stability.　The　addition　of　3　wt　%　HUMCS　increased　the　LOI　from　31.4　to　33.0.　The　addition　of　HUMCS　at　a　low　additive　amount　reduced　the　peak　heat-release　rate,　total　heat　release,　and　heat-release　capacity　obviously.　Furthermore,　scanning　electron　micrographs　of　char　residues　revealed　that　HUMCS　could　prevent　the　IFR-PP　composites　from　forming　a　dense　and　compact　multicell　char,　which　could　effectively　protect　the　substrate　material　from　combusting.　(C)　2014　Wiley　Periodicals,　Inc.