Cement　has　been　widely　used　as　a　structural　material　in　many　underground　projects,　and　these　projects　often　face　high-　or　ultra-high-temperature　environments,　leading　to　the　deterioration　of　the　mechanical,　porosity,　and　permeability　properties　of　set　cement,　thereby　increasing　the　risk　of　instability　of　underground　structures.　In　response　to　this,　two　new　temperature-resistant　cement　slurry　systems　were　designed.　Experiments　were　conducted　on　the　changes　in　porosity　and　permeability　of　set　cement　after　thermal　treatment　using　low-field　nuclear　magnetic　resonance　technology　(NMR),　visual　studies　of　pore　and　crack　development　were　carried　out　using　the　argon-ion　polishing　field　emission　scanning　electron　microscopy　(FE-SEM)　and　computed　tomography　(CT)　methods.　The　research　results　show　that　as　the　thermal　treatment　temperature　continued　to　rise,　the　compressive　strength　first　increased　(25　degrees　C-200　degrees　C)　and　then　decreased　(200　degrees　C-600　degrees　C).　The　porosity　of　the　set　cement　first　decreased　(25　degrees　C-115　degrees　C)　and　then　increased　(115　degrees　C-600　degrees　C),　and　the　penetration　first　slowly　increased　(25　degrees　C-400　degrees　C)　and　then　rapidly　increased　(400　degrees　C-600　degrees　C).　Visualization　experiments　were　conducted　on　micro-cracks　and　the　pore　distribution　of　the　set　cement　under　high-　and　ultra-high-temperatures,　which　proved　the　evolution　law　of　these　characteristic　parameters.　The　research　results　have　vital　reference　significance　for　the　protection　of　the　structural　stability　of　cement　components　when　encountering　high-temperature　environments.