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　pi-pi　interaction　between　adjacent　layers　in　TCP-COF.　The　resultant　ultrathin　nanosheets　significantly　reinforce　catalytic　activity,　achieving　a　photocatalytic　H2O2　production　rate　of　3077　mu　mol　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.