Photocatalytic　reduction　of　CO2　to　value-added　fuel　has　been　considered　to　be　a　promising　strategy　to　reduce　global　warming　and　shortage　of　energy.　Rational　design　and　synthesis　of　catalysts　to　maximumly　expose　the　active　sites　is　the　key　to　activate　CO2　molecules　and　determine　the　reaction　selectivity.　Herein,　we　synthesize　a　well-defined　copper-based　boron　imidazolate　cage　(BIF-29)　with　six　exposed　mononuclear　copper　centers　for　the　photocatalytic　reduction　of　CO2.　Theoretical　calculations　show　a　single　Cu　site　including　weak　coordinated　water　delivers　a　new　state　in　the　conduction　band　near　the　Fermi　level　and　stabilizes　the　*COOH　intermediate.　Steady-state　and　time-resolved　fluorescence　spectra　show　these　Cu　sites　promote　the　separation　of　electron–hole　pairs　and　electron　transfer.　As　a　result,　the　cage　achieves　solar-driven　reduction　of　CO2　to　CO　with　an　evolution　rate　of　3334　μmol　g−1　h−1　and　a　high　selectivity　of　82.6　%.　©　2019　Wiley-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim