Ultrathin　Co0.85Se　nanosheets　were　prepared　via　a　facile　solvothermal　method,　which　successfully　overcome　the　disadvantage　of　insufficient　active　sites　in　bulk　Co0.85Se　by　reducing　its　thickness　into　the　atomic　scale.　Co0.85Se　nanosheets　have　been　found　to　be　efficient　cocatalysts　for　photocatalytic　CO2　reduction　with　TON　as　high　as　58　in　1　h　reaction　under　visible-light　irradiation.　Compared　with　bulk　Co0.85Se　blocks,　the　ultrathin　nanosheet　samples　exhibited　nearly　2.5　times　higher　activity　in　CO2　photoreduction.　The　metal-like　character　and　ultrathin　2D　structure　of　Co0.85Se　nanosheets　support　the　superb　charges　transfer　and　rich　CO2　adsorption　sites　exposure　for　surface　catalysis.　A　relationship　between　the　surface　structure　and　the　cocatalytic　activity　was　established.　These　results　not　only　demonstrate　the　potential　of　a　notable,　affordable　and　earth-abundant　CO2　photoreduction　cocatalyst　based　on　ultrathin　Co0.85Se　nanosheets　but　also　provide　a　guidance　in　the　exploration　of　excellent　active　and　durable　cocatalysts/catalysts　to　replace　noble　metals　for　artificial　photosynthesis.