Photocatalytic conversion of CO2 into valued chemicals is limited by the CO2 adsorption and activation capability on catalyst surface. In this work, Sn element was successfully doped in ultrathin BiOCl with oxygen vacancies. The dipole moment of catalyst efficient increased due to the Sn doping, leading to a stronger internal electric field. In addition, the surface frustrated Lewis pairs were simultaneously constructed near the oxygen vacancies due to Sn dopant. The formed region of frustrated Lewis pairs-oxygen vacancies as active sites enable an improved CO2 adsorption, and the enhanced internal electric field intensity promoted the charge carrier separation and transfer, leading to the enhanced CO2 reduction performances. This work provides light on enhancing internal electric field by increasing dipole moment and presents feasible tunability of catalytic active sites at atom-scale, providing fundamental insights for understanding of mechanism of element doping in catalyst for improving CO2 photoreduction performance.