Environmental　factors　have　a　significant　impact　on　the　performance　of　cement-based　materials.　As　a　new　type　of　low-carbon　cementitious　material,　the　performance　of　magnesium　silicate　hydrate　cement　(MSHC)　under　different　environmental　humidity　conditions　is　crucial　for　its　application.　This　study　aimed　to　investigate　the　effects　of　environmental　humidity　on　the　hardening　properties　of　MSHC.　Compressive　strength　tests　of　MSHC　were　conducted　under　four　different　relative　humidity　(RH)　conditions　(RH　of　33%,　58%,　75%,　and　97%)　and　water　curing　(RH-w)　condition.　Microanalysis　techniques　were　employed　to　reveal　the　strength　mechanisms　of　MSHC　under　different　environmental　humidity　conditions.　The　results　showed　that　MSHC　exhibited　the　optimal　mechanical　performance　when　RH　was　75%,　reaching　42.29　MPa.　When　RH　was　58%,　the　compressive　strength　of　MSHC　was　close　to　the　optimal　level,　which　was　41.02　MPa.　However,　under　other　humidity　conditions　(RH33,　RH97,　and　RH-w),　the　compressive　strength　of　MSHC　was　much　lower　than　the　optimal　level,　decreasing　by　30.93%,　41.90%,　and　63.66%,　respectively.　Pore　structure　analysis　showed　that　the　volume　of　detrimental　pores　(　＞100　nm)　in　the　sample　was　the　lowest　when　RH　was　75%.　Compared　with　RH75,　the　volume　of　detrimental　pores　in　RH33,　RH58,　RH97,　and　RH-w　increased　by　20.47%,　9.83%,　36.77%,　and　156.92%,　respectively.　In　addition,　the　variation　of　environmental　humidity　resulted　in　different　polymerization　levels　in　the　M　-S　-H　gel,　and　higher　degrees　of　polymerization　were　observed　when　RH　was　58%　and　75%.