In　this　paper,　the　variation　of　compressive　strength　of　cement　paste　specimens　under　external　sulfate　attack　was　studied.　Then　the　microstructure　characteristics　of　the　cement　attacked　by　sodium　sulfate　solution　was　observed　by　the　Scanning　Electron　Microscope　(SEM)　test.　Subsequently,　an　in-suit　X-ray　CT　testing　technology　was　used　to　monitor　the　change　of　pore　structure　in　cement　paste　specimens　under　different　exposure　time,　and　the　Avizo　software　was　used　to　calculate　the　porosity　of　cement　paste　specimens　and　porosities　of　different　depth　zones　in　cement　paste　specimen.　The　results　show　that　the　compressive　strength　increases　firstly　and　then　decreases.　Owing　to　that　ettringites　are　produced　in　cement　paste　specimens,　and　the　number　and　the　volume　of　ettringites　increase　with　the　increase　of　exposure　time.　Leading　to　that　the　porosity　of　cement　paste　specimens　decreases　first　and　then　increases　with　the　increase　of　exposure　time.　And　the　porosities　of　different　depth　zones　undergo　a　process　of　decreasing　first　and　then　increasing　from　surface　to　interior　in　cement　paste　specimens.　It　indicates　that　the　deterioration　process　is　from　the　surface　to　the　interior　with　the　increase　of　exposure　time.