The　tricalcium　silicate　(C3S)　paste　with　high　electromagnetic　transmission　performance　is　prepared　by　addition　of　nanocrystalline　cellulose　(NCC),　and　the　enhancement　mechanism　of　electromagnetic　transmission　performance　is　revealed.　The　water　to　cement　ratio　(W/C)　and　the　contents　of　NCC　are　0.5　and　1　wt%-4wt%,　respectively.　The　results　show　that　the　electromagnetic　transmissivity　of　the　C3S　paste　improves　with　an　increase　in　NCC　content.　At　the　frequency　of　3.94　GHz,　when　the　NCC　contents　are　3　wt%　and　4　wt%,　the　electromagnetic　transmission　coefficient　increases　by　113.85%　and　153.6%,　respectively.　The　enhancement　mechanisms　of　electromagnetic　transmission　performance　of　NCC-C3S　pastes　are　as　follows:　(1)　the　polymerization　degree　of　C-S-H　and　the　Van　der　Waals　force　between　the　globules　increase;　(2)　the　total　volume　proportions　of　low　electromagnetic　loss　C3S　and　Ca(OH)2　increase;　(3)　the　conductivity　of　C3S　pore　solution　and　the　volume　proportions　of　water　decrease;　(4)　the　pore　volume　proportion　of　the　non-harmful　pores　for　electromagnetic　energy　loss　increases.