Ultrathin　polymeric　carbon　nitride　nanosheets　(PCN-NSs)　are　considered　as　a　promising　semiconductor　photocatalyst　for　CO2　reduction,　but　the　achieved　efficiency　is　still　unsatisfactory.　Herein,　a　methanol-mediated　steam　delaminating　strategy　is　demonstrated　to　fabricate　distorted　PCN-NSs　(d-PCN-NSs)　with　activated　n-π*　electron　transition　and　favorite　textural　properties.　The　ultrathin　d-PCN-NSs　can　absorb　optical　spectrum　extending　to　700　nm,　accelerate　separation　and　transportation　of　photoinduced　charge　carriers,　and　deliver　rich　disclosed　active　sites　for　CO2　adsorption/activation.　Compared　to　that　of　pristine　PCN　and　plane　PCN-NSs,　the　CO2-to-CO　conversion　activity　of　d-PCN-NSs　is　enhanced　by　35.7　and　4.4　times,　respectively,　displaying　a　high　apparent　quantum　efficiency　(AQE)　of　3.8%　under　photoirradiation　at　420　nm.　Of　note,　the　d-PCN-NSs　photocatalyst　can　still　catalyze　a　CO2　photoreduction　reaction　under　660　nm　illumination.　Based　on　the　key　catalytic-active　intermediate　(i.e.,　COOH*)　determined　by　in-situ　diffuse　reflectance　infrared　Fourier　transform　spectroscopy,　the　possible　mechanism　of　the　tandem　CO2　photoreduction　catalysis　is　proposed.　©　2021　Elsevier　Inc.