As　a　universal　polymer　material,　polystyrene　(PS)　is　widely　applied　in　electrical　devices　and　construction.　Thus,　it　is　necessary　to　improve　the　flame　retardancy　and　electromagnetic　shielding　properties　of　PS　material.　In　this　work,　PS/silicon-wrapped　ammonium　polyphosphate/Inorganic　acid-treated　multi-walled　carbon　nanotubes　composites　(PS/SiAPP/aMWCNT,　abbreviated　as　PAC)　were　prepared　via　methods　of　filtration-induced　assembly　and　hot-pressing.　Morphology　and　structure　characterization　demonstrated　that　SiAPP　and　aMWCNT　had　good　dispersion　in　PS　and　excellent　compatibility　with　the　PS　matrix.　Thermogravimetric　analysis　revealed　that　the　addition　of　aMWCNT　to　PS　improved　its　thermal　stability　and　carbon-forming　characteristics.　The　peak　heat　release　rate,　the　peak　carbon　monoxide　production　rate,　and　the　peak　smoke　production　rate　of　the　PAC10　composite　decreased　by　53.7%,　41.9%,　and　45.5%,　respectively,　while　its　electromagnetic　shielding　effectiveness　reached　12　dB.　These　enhancements　were　attributed　to　the　reason　that　SiAPP　and　aMWCNT　synergistically　catalyzed　the　char　generation　and　SiAPP　produced　free　radical　scavengers　and　numbers　of　incombustible　gases,　which　could　decrease　the　oxygen　concentration　and　retard　the　combustion　reaction.　Therefore,　the　assembled　PS/SiAPP/aMWCNT　system　provides　a　new　pathway　to　improve　the　flame　retardant　and　electromagnetic　shielding　properties　of　PS.