Covalent　organic　frameworks　(COFs)　containing　dioxin-linkages　are　highly　valued　for　their　exceptional　chemical　stability,　which　is　essential　for　practical　use.　However,　research　on　dioxin-based　COFs　remains　limited.　Herein,　a　unique　nonplanar　2D　COF,　designated　as　TCP-COF,　constructed　from　catechol-porphyrin　units　interconnected　by　1,4-dioxin　bonds,　exhibiting　a　staggered　AAA　stacking　pattern,　is　presented.　Remarkably,　TCP-COF　can　undergo　in　situ　exfoliation　to　produce　ultrathin　2D　nanosheets　when　it　is　utilized　as　a　photocatalyst　for　hydrogen　peroxide　(H2O2)　generation　in　water　and　air,　without　the　need　for　additives.　This　exfoliation　process　is　primarily　driven　by　the　distortion　of　porphyrin　units　and　weak　π–π　interaction　between　adjacent　layers　in　TCP-COF.　The　resultant　ultrathin　nanosheets　significantly　reinforce　catalytic　activity,　achieving　a　photocatalytic　H2O2　production　rate　of　3077 µmol　g−1 h−1.　The　mechanism　underlying　H2O2　photosynthesis　is　further　explored　through　a　combination　of　experimental　analyses　and　theoretical　calculations.　This　study　provides　valuable　insights　for　the　development　of　efficient　COF-based　photocatalysts　for　H2O2　evolution.　©　2025　Wiley-VCH　GmbH.