The solar-driven reduction of CO₂ into valuable products is a promising method to alleviate global environmental problems and energy crises. However, the low surface charge density limits the photocatalytic conversion performance of CO₂. Herein, a polymeric carbon nitride (PCN) photocatalyst with Zn single atoms (Zn₁/CN) was designed and synthesized for CO₂ photoreduction. The results of the CO₂ photoreduction studies show that the CO and CH₄ yields of Zn₁/CN increased fivefold, reaching 76.9 and 22.9 μmol/(g·h), respectively, in contrast to the unmodified PCN. Ar⁺ plasma-etched X-ray photoelectron spectroscopy and synchrotron radiation-based X-ray absorption fine structure results reveal that Zn single atom is mainly present in the interlayer space of PCN in the Zn–N₄ configuration. Photoelectrochemical characterizations indicate that the interlayer Zn–N₄ configuration can amplify light absorption and establish an interlayer charge transfer channel. Light-assisted Kelvin probe force microscopy confirms that more photogenerated electrons are delivered to the catalyst surface through interlayer Zn–N₄ configuration, which increases its surface charge density. Further, in-situ infrared spectroscopy combined with density functional theory calculation reveals that promoted surface charge density accelerates key intermediates (*COOH) conversion, thus achieving efficient CO₂ conversion. This work elucidates the role of internal single atoms in catalytic surface reactions, which provides important implications for the design of single-atom catalysts.