The selective reduction of carbon dioxide (CO₂) into fuels and chemicals is expected to play a key role in achieving a carbon–neutral sustainable energy economy. Activation of CO₂ is the most important step in this process. As the electron transfer to CO₂ is the rate-limiting step for CO₂ activation, it is significant to modulate the electronic structure of CO₂ reduction catalysts to enhance their activities. However, the intrinsic relationships between electronic properties of catalysts and their catalytic activities remains somewhat unclear, which limits the rational design of highly-efficient CO₂ reduction catalysts. Herein, a catalyst-insulator–metal system consisting of Al as an electron-donor material was designed to modulate the electronic structure of cobalt oxide (Co₃O₄) catalyst. The electrons in Al can be efficiently tunneled to Co₃O₄ across an ultrathin self-formed Al₂O₃ insulating layer. Experimental and theoretical results undoubtedly confirm that high electron density on Co₃O₄ favours the adsorption and activation of CO₂, while lowers the energy barrier for the formation of COOH* and especially lowers the desorption energy barrier of CO* intermediates, thereby significantly accelerating the reaction kinetics of CO₂-to-CO photoreduction. The turnover frequency per Co atom of Co₃O₄/Al₂O₃-Al is much higher than that of Co₃O₄. The apparent quantum yield (AQY) of Co₃O₄/Al₂O₃-Al catalyst highly reaches 3.8% at 420 nm, which is superior to those of most reported catalysts. Besides, the increase in electron density on Co₃O₄ can effectively inhibit the competing hydrogen evolution reaction. The selectivity of CO increases from 57.9% for Co₃O₄ to 82.4% for Co₃O₄/Al₂O₃-Al. Significantly, the catalytic efficiency can be rationally tuned through controlling the content and particle size of Al. This work stablishes the links between the electronic structure of catalysts and their catalytic activities toward CO₂ reduction reaction. Moreover, this Al₂O₃-Al structure reported here, may be used as an unprecedented platform for the electronic effect study of other heterogeneous catalysts.