Photothermal catalytic CO2 reduction to ethanol is a key pathway for carbon cycle utilization, but its development is limited by the bottlenecks of product selectivity regulation and low C–C coupling efficiency. In this study, a graphene oxide (rGO)-supported high-density Cu/Cu2O heterojunction catalyst was constructed via a “one-pot hydrothermal-high-temperature hydrogen calcination” strategy, leveraging the confinement and electronic modulation effects of “rGO fences” to achieve a significant leap in catalytic performance. Charge density difference and density of states (DOS) analyses reveal that a strong built-in electric field directed from Cu to Cu2O is formed at the heterojunction interface, which efficiently promotes the separation and transfer of charge carriers and optimizes the adsorption of intermediates by regulating the d-band center. Under light irradiation, the localized surface plasmon resonance (LSPR) effect of Cu synergizes with the built-in electric field to enhance the “hot electron” injection efficiency. In situ Fourier transform infrared spectroscopy (in situ FT-IR) and density functional theory (DFT) calculations confirm that the rate-determining step (RDS) energy barrier of the C–C asymmetric coupling pathway of *CO/*CHO at the interface is only 0.92 eV, which is significantly lower than that of side reaction pathways. Under optimal reaction conditions (160 °C, 2 MPa, CO2/H2 = 1:3), the catalyst achieves an ethanol yield of 3250 μmol g−1 h−1 and a liquid-phase selectivity of 93%, providing new insights for the design of efficient catalysts for CO2 conversion to C2+ products.