Probing　the　dynamic　evolution　of　catalyst　structure　and　chemical　state　under　operating　conditions　is　highly　important　for　investigating　the　reaction　mechanism　of　catalysis　more　in　depth,　which　in　turn　advances　the　rational　design　of　redox　catalysis　in　using　renewable　energy　to　produce　fuels.　Herein,　the　evolution　of　atomically　dispersed　Cu　species　supported　by　mesoporous　TiO2　(mTiO2)　during　the　in　situ　photocatalytic　reduction　of　CO2　with　H2O　to　valuable　solar　fuels　has　been　reported.　The　results　unveil　that　the　initial　atomically　dispersed　Cu(II)　undergoes　reduction　to　Cu(I)　and　ultimately　to　Cu(0);　the　Cu(I)/Cu(0)　mixture　is　proposed　to　be　more　effective　for　CH4　formation.　In　addition,　the　enhanced　CO2　adsorption　ability　benefited　from　the　structural　advantage　of　mTiO2　and　the　elevated　charge　carrier　transfer　synergistically　contributes　to　the　CO2　photoreduction.　It　is　anticipated　that　this　work　would　guide　the　rational　design　of　Cu-based　light-harvesting　catalysts　for　artificial　CO2　reduction　to　value-added　feedstocks　and　inspire　further　interest　in　using　in　situ　techniques　to　study　the　structure-activity　interplay　of　photocatalysts　under　operating　reaction　conditions.　©　2019　American　Chemical　Society.