The quaternary AgInP2S6 atomic layer with the thickness of ~ 0.70 nm were successfully synthesized through facile ultrasonic exfoliation of the corresponding bulk crystal. The ultrathin sheet exhibits efficiently photocatalytic conversion of CO2 into CO as a major product and minority of CH4 and C2H4 in the presence of water vapor. The sulfur defect engineering on this atomic layer through a H2O2 etch process can excitingly enable to change the CO2 photoreduction reaction pathway to steer dominant generation of ethene (C2H4) important chemical with the yield-based selectivity reaching ~73% and the electron-based selectivity as high as ~89%, and the quantum yield of 0.51% at wavelength of 415 nm. Both DFT calculation and in-situ FTIR demonstrate as the introduction of S vacancies in AgInP2S6 causes the charge accumulation on the Ag atoms near the S vacancies, the exposed Ag sites can thus effectively capture the forming *CO molecules, making the catalyst surface enrich with key reaction intermediates to lower the C-C binding coupling barrier, which facilitates the production of C2H4. Surface photovoltage measurement confirms that atomically ultrathin structure of the exfoliated AgInP2S6 can shorten the transfer distance of charge carriers from the interior onto the surface, thus decrease the recombination in body and improve the catalytic efficiency. This work may provide fresh insights into the design of atomically thin photocatalyst framework for CO2 reduction and establish an ideal platform for reaffirming the versatility of defect engineering in tuning catalytic activity and selectivity.