This　paper　investigated　the　electromagnetic　shielding　properties　and　the　mechanisms　of　tricalcium　silicate　(C3S)　paste　with　three　water-cement　(W/C)　ratios　after　long-term　curing.　The　C3S　pastes　with　three　W/C　ratios　were　tested　by　XRD　and　other　tests　for　the　qualitative　and　quantitative　analysis　of　the　individual　phases.　The　electromagnetic　shielding　properties　and　electromagnetic　parameters　of　the　specimens　were　obtained　using　the　waveguide　method.　Finally,　the　microscopic　mechanism　of　the　variation　of　electromagnetic　shielding　properties　of　C3S　paste　affected　by　the　W/C　ratio　was　dissected　based　on　the　individual　phases.　The　results　show　that　the　volume　fraction　of　pore,　calcium　silicate　hydrate　(C-S-H)　gel,　calcium　hydroxide　(CH),　and　calcite　(CaCO3)　in　the　specimen　increases　with　the　rise　of　the　W/C　ratio,　and　the　unhydrated　C3S　decreases;　the　electromagnetic　shielding　of　the　paste　specimen　increases;　the　complex　relative　permittivity　gradually　increases.　The　mechanism　of　the　W/C　ratio　affecting　the　electromagnetic　shielding　performance　of　C3S　paste　is　that,　on　the　one　hand,　both　the　volume　fraction　of　C-S-H　and　the　relative　permittivity　in　the　solid　phase,　which　have　the　greatest　influence　on　the　electromagnetic　shielding　performance,　gradually　increase　with　the　increase　of　W/C　ratio.　On　the　other　hand,　as　the　W/C　ratio　increases,　the　volume　fraction　and　area　fraction　of　the　mesopores　in　the　specimen　increase,　which　increases　the　internal　loss　of　electromagnetic　waves.