Photoelectrochemical (PEC) reduction of CO2 with H2O not only provides an opportunity for reducing net CO2 emissions but also produces value-added chemical feedstocks and fuels. Syngas, a mixture of CO and H-2, is a key feedstock for the production of methanol and other commodity hydrocarbons in industry. However, it is challenging to achieve efficient and stable PEC CO2 reduction into syngas with controlled composition owing to the difficulties associated with the chemical inertness of CO2 and complex reaction network of CO2 conversion. Herein, by employing a metal/oxide interface to spontaneously activate CO2 molecule and stabilize the key reaction intermediates, we report a benchmarking solar-to-syngas efficiency of 0.87% and a high turnover number of 24 800, as well as a desirable high stability of 10 h. Moreover, the CO/H-2 ratios in the composition can be tuned in a wide range between 4:1 and 1:6 with a total unity Faradaic efficiency. On the basis of experimental measurements and theoretical calculations, we present that the metal/oxide interface provides multifunctional catalytic sites with complementary chemical properties for CO2 activation and conversion, leading to a unique pathway that is inaccessible with the individual components. The present approach opens new opportunities to rationally develop high-performance PEC systems for selective CO2 reduction into valuable carbon-based chemicals and fuels.