The　deposition　of　a　two-dimensional　(2D)　atomic　nanosheet　on　a　metal　surface　has　been　considered　as　a　new　route　for　tuning　the　molecule-metal　interaction　and　surface　reactivity　in　terms　of　the　confinement　effect.　In　this　work,　we　use　first-principles　calculations　to　systematically　explore　a　novel　nanospace　constructed　by　placing　a　2D　graphitic　carbon　nitride　(g-C3N4)　nanosheet　over　a　Pt(111)　surface.　The　confined　catalytic　activity　in　this　nanospace　is　investigated　using　CO　oxidation　as　a　model　reaction.　With　the　inherent　triangular　pores　in　the　g-C3N4　overlayer　being　taken　advantage　of,　molecules　such　as　CO　and　O2　can　diffuse　to　adsorb　on　the　Pt(111)　surface　underneath　the　g-C3N4　overlayer.　Moreover,　the　mechanism　of　intercalation　is　also　elucidated,　and　the　results　reveal　that　the　energy　barrier　depends　mainly　on　the　properties　of　the　molecule　and　the　channel.　Importantly,　the　molecule-catalyst　interaction　can　be　tuned　by　the　g-C3N4　overlayer,　considerably　reducing　the　adsorption　energy　of　CO　on　Pt(111)　and　leading　to　enhanced　reactivity　in　CO　oxidation.　This　work　will　provide　important　insight　for　constructing　a　promising　nanoreactor　in　which　the　following　is　observed:　The　molecule　intercalation　is　facile;　the　molecule-metal　interaction　is　efficiently　tuned;　the　metal-catalyzed　reaction　is　promoted.　©　2017　American　Chemical　Society.