The　weak　long-term　photocatalytic　performance　of　photocatalytic　cementitious　materials　under　real-life　service　conditions　has　always　been　one　of　the　main　factors　restricting　their　large-scale　application.　A　novel　stable　threedimensional　structure　of　S-g-C3N4/MgAl-LDH　was　constructed　by　composing　S-g-C3N4　with　MgAl-LDH,　aiming　to　improve　its　compatibility　with　cement　mortar　to　achieve　ideal　long-term　photocatalytic　performance　under　natural　exposure.　Taking　S-g-C3N4　as　reference,　the　possible　contributions　of　MgAl-LDH　on　improving　the　mechanical　strength　and　long-term　photocatalytic　performance　of　mortar　was　analyzed　by　XRD,　TG/DTG,　SEM-EDX　and　isothermal　calorimetry.　The　result　shows　that　since　the　high　specific　surface　area　and　refinement　of　MgAl-LDH,　the　dense　surface　morphology　and　fewer　surface　defects　jointly　promote　the　improvement　of　strength.　When　the　content　of　S-g-C3N4/MgAl-LDH　is　6　%,　the　28d　compressive　strength　of　composite　mortar　is　excellent,　which　is　28.7　%　higher　than　the　pure　cement　mortar.　More　importantly,　under　the　stimulation　of　the　calcium-rich　layer　on　the　surface　of　S-g-C3N4/MgAl-LDH,　the　accumulation　of　a　large　amount　of　gelling　hydration　products　is　beneficial　to　the　stable　long-term　photocatalytic　performance　of　S-g-C3N4/MgAl-LDH　doped　mortar.　After　180　days　of　natural　exposure,　the　NOx　removal　rate　of　the　CMSL-8　%　sample　can　still　remain　at　68.3　%　of　the　original　value　(328　mu　mol　center　dot　m(-2)center　dot　h(-1)).　This　research　provides　a　new　theoretical　basis　for　photocatalytic　cementitious　materials　to　achieve　ideal　long-term　photocatalytic　performance.