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A CsPbBr3@Ag-C3N4 ternary heterojunction photocatalyst with g-C3N4 as ballistic electron transport "tunnel" is designed and synthesized. The photogenerated electrons of CsPbBr3 dots can be efficiently transferred to Ag nanoparticles via g-C3N4 sheets, resulting in efficient CO2 reduction. The structured design of catalyst provides a novel insight for the development of perovskite-based photocatalysts. [Display omitted] • CsPbBr 3 @Ag-C 3 N 4 ternary heterojunction photocatalyst is designed and synthesized. • The photogenerated electrons of CsPbBr 3 QDs can be efficiently transferred to Ag nanoparticles via g-C 3 N 4 as the electron transport tunnel. • The CsPbBr 3 @Ag-C 3 N 4 ternary photocatalyst achieves a conversion rate of 19.49 μmol·g−1·h−1 with almost 100 % CO selectivity. • In situ DRIFT and DFT calculations reveal the photocatalytic mechanism for CO 2 reduction. Perovskite CsPbBr 3 quantum dot shows great potential in artificial photosynthesis, attributed to its outstanding optoelectronic properties. Nevertheless, its photocatalytic activity is hindered by insufficient catalytic active sites and severe charge recombination. In this work, a CsPbBr 3 @Ag-C 3 N 4 ternary heterojunction photocatalyst is designed and synthesized for high-efficiency CO 2 reduction. The CsPbBr 3 quantum dots and Ag nanoparticles are chemically anchored on the surface of g-C 3 N 4 sheets, forming an electron transfer tunnel from CsPbBr 3 quantum dots to Ag nanoparticles via g-C 3 N 4 sheets. The resulting CsPbBr 3 @Ag-C 3 N 4 ternary photocatalyst, with spatial separation of photogenerated carriers, achieves a remarkable conversion rate of 19.49 μmol·g−1·h−1 with almost 100 % CO selectivity, a 3.13-fold enhancement in photocatalytic activity as compared to CsPbBr 3 quantum dots. Density functional theory calculations reveal the rapid CO 2 adsorption/activation and the decreased free energy (0.66 eV) of *COOH formation at the interface of Ag nanoparticles and g-C 3 N 4 in contrast to the g-C 3 N 4 , leading to the excellent photocatalytic activity, while the thermodynamically favored CO desorption contributes to the high CO selectivity. This work presents an innovative strategy of constructing perovskite-based photocatalyst by modulating catalyst structure and offers profound insights for efficient CO 2 conversion. [ABSTRACT FROM AUTHOR]