To　further　improve　the　efficiency　of　intumescent　flame　retardant　(IFR)　system　consisting　of　ammonium　polyphosphate　and　charring-foaming　agent,　MoO3-ZrO2　solid　acid　is　used　as　an　adjuvant.　The　influence　of　MoO3-ZrO2　on　the　thermal　decomposition,　reaction　to　small　flame　and　fire　behavior　of　polypropylene　(PP)/IFR　composite　is　investigated.　The　incorporation　of　1　wt%　solid　acid　results　in　the　enhanced　limiting　oxygen　index　(LOI)　and　fire　safety　of　PP/IFR　composite,　while　decreasing　its　peak　heat　release　rate　from　431　to　370　kW/m2.　Flame　retardant　mechanism　is　proposed　based　on　the　analysis　of　thermal　decomposition　behavior　and　char　residue.　The　yield　and　graphitization　degree　of　char　residue　are　improved　by　MoO3-ZrO2.　The　release　of　CO2　from　thermal　decomposition　of　IFR　is　accelerated　by　this　solid　acid.　This　work　provides　a　new　path　to　designing　highly　fire　retardant　polymer　composites.　©　2018　Elsevier　B.V.