Semiconductor-based photocatalytic CO2 conversion is a promising and sustainable avenue in response to the anthropogenic climate change and imminent energy crisis, which however is unavoidably impeded by the limited photoabsorption, undesirable recombination of photogenerated charge carriers and insufficient surface active sites on semiconductors. In this study, all these challenges were overcome by selectively and chemically assembly of oxygen-deficient Bi2WO6 nanosheets onto BiOI nanosheets, forming a novel surface defect-engineered 2D/2D motif with builtin nanoscale p-n heterojunctions. This rational cascade configuration with internal electric field renders ultrafast directional migration and spatial separation of photogenerated charge carriers. Meanwhile, the oxygen vacant sites with abundant trapped electrons serve as the active sites for CO2 reduction and extend the light absorption of the photocatalytic system to NIR region. Combining these propitious properties, our delineated nanoscale p-n heterojunction on the basis of 2D/2D assembly of surface defect-engineered nanosheets presents a new and unprecedented concept for effective generation of charge carriers, directional steering of charge flow and manipulation of surface active sites, which cooperatively lead to superior photocatalytic performance. Notably, our developed oxygen-deficient Bi2WO6/BiOI binanosheets exhibit a remarkably high production yield of CH4, which represents the state-of-the-art visible light-driven CH4 production activity among all the existing 2D BiOI-based and Bi2WO6-based composites.