External　wall　composite　insulation　materials　have　shown　great　potential　for　energy　saving　and　emission　reduction　in　the　construction　industry,　but　cement-based　material　as　one　of　the　external　wall　composite　insulation　materials　has　the　problem　of　high　thermal　conductivity.　Herein,　a　composite　insulation　material　was　prepared　using　the　press　method　by　expanded　polystyrene　(EPS)　and　cement　of　different　calcium　silicate　content.　The　addition　of　porous　calcium　silicate　provides　nucleation　sites　for　cement　hydration　to　promote　hydration　reaction　and　increase　the　porosity　of　the　inorganic　cementitious　phase　from　75　%　to　85　%　and　refine　the　average　pore　size　from　2826.09　nm　to　421.31　nm.　The　cement-based　composite　insulation　material　which　the　calcium　silicate　content　is　20　%　has　0.0423　W/(m　center　dot　K)　in　the　thermal　conductivity,　which　is　23.1　%　lower　than　the　noncalcium　silicate　content　of　cement-based　composite　insulation　material　(0.055　W/(m　center　dot　K)).　The　simulation　analysis　proves　that　the　improved　insulating　properties　are　attributed　to　the　increased　interface　between　solid-phase　and　gas　phase,　thinner　heat　transfer　walls,　and　longer　heat　transfer　paths　due　to　the　pore　structure　changes　in　the　inorganic　cementitious　phase.　This　proposed　composite　insulation　material　provides　a　new　choice　for　external　wall　insulation.