This　paper　explores　the　impact　of　sand　content　and　particle　size　on　the　electromagnetic　transmission　performance　(ETP)　of　mortar　samples.　This　study　involves　designing　mortar　samples　with　a　water-cement　ratio　of　0.4　and　varying　sand　volume　content　from　0　%　to　50　%.　Additionally,　a　mesoscopic　finite　element　model　is　established　to　analyze　the　electromagnetic　transmission　behavior.　The　findings　indicate　that　the　particle　size　and　content　of　sand　influence　the　multiple　reflections　behavior　of　electromagnetic　waves,　while　the　particle　size　of　sand　has　almost　no　effect　on　the　electromagnetic　transmission　coefficient　and　dielectric　response.　Although　sand　enhances　the　ETP　of　cement　paste,　its　impact　is　limited.　Furthermore,　the　study　reveals　that　the　distribution　of　the　electromagnetic　field　remains　continuous　regardless　of　sand　particle　size　or　content,　and　is　unaffected　by　the　differing　dielectric　properties　of　sand　and　cement　paste　phases.　However,　the　induced　current　generated　by　sand　and　other　phases　differs,　leading　to　noticeable　variations　in　dielectric　loss.　Mesoscopic　simulation　results　clearly　demonstrate　that　cement　paste　is　the　primary　contributor　to　electromagnetic　energy　loss.