Developing　polymer　nanocomposites　with　high　thermal　stability　and　excellent　flame　retardant　performances　at　low　loadings　still　remains　a　great　challenge.　Here,　we　constructed　multilayered　polystyrene　(PS)　nanocomposites　consisting　of　graphitic　carbon　nitride　nanosheets　(g-C3N4　NSs)　and　reduced　graphene　oxide　(RGO)　using　the　layer　by　layer　assembly　technique.　Microstructure　analysis　demonstrated　the　successfully　alternative　assembly　of　g-C3N4　NSs　and　RGO　onto　PS　spheres　through　electrostatic　interaction.　The　ternary　PS　nanocomposites　showed　the　highest　thermal　stability.　For　instance,　the　temperature　at　maximum　decomposition　rate　and　the　residual　yield　were　increased　by　15　degrees　C　and　5.89　wt%,　respectively　for　PS　nanocomposite　with　one　number　of　g-C3N4/RGO　bilayers　(PS/CG1).　Furthermore,　the　PS/CG1　exhibited　dramatically　lowest　release　of　total　pyrolysis　gaseous　products　and　the　peak　of　heat　release　rate,　indicative　of　the　highest　fire　safety.　Fire　safety　enhancement　was　attributed　to　the　formation　of　compact　network　barriers　that　retarded　the　heat　and　mass　transfer　between　combustion　zone　and　underlying　PS　matrix.