This study explores the synthesis of well-defined diatomic catalysts (DACs) for efficient photosynthesis of C₂H₄ from CO₂. Traditional bottom-up strategies for DACs synthesis have limitations due to the random distribution of heteronuclear atoms, which hinders catalytic performance optimization and reaction mechanism exploration. To address this, an up-bottom ion-cutting architecture is proposed to fabricate well-defined CuAu DACs (CuAu-DAs) decorated on TiO₂. The compact heteroatomic spacing (2.3 Å) of CuAu-DAs-TiO₂ results in a significantly low C-C coupling energy barrier, leading to high sustainability in C₂H₄ production. The discovery of this novel up-bottom strategy provides a new approach for optimizing catalytic performance and understanding the synergistic catalytic mechanism of heteroatom sites. The study also investigates the photocatalytic performance and stability of CuAu-DAs-TiO₂, demonstrating superior activity and stability in C₂H₄ production compared to other catalysts. The mechanism of the photocatalytic performance is elucidated through in situ DRIFTS and DFT calculations, revealing the synergistic effect of Cu-SAs and Au-SAs in promoting *CO formation and conversion, as well as suppressing catalyst deactivation under high-*CO conditions.This study explores the synthesis of well-defined diatomic catalysts (DACs) for efficient photosynthesis of C₂H₄ from CO₂. Traditional bottom-up strategies for DACs synthesis have limitations due to the random distribution of heteronuclear atoms, which hinders catalytic performance optimization and reaction mechanism exploration. To address this, an up-bottom ion-cutting architecture is proposed to fabricate well-defined CuAu DACs (CuAu-DAs) decorated on TiO₂. The compact heteroatomic spacing (2.3 Å) of CuAu-DAs-TiO₂ results in a significantly low C-C coupling energy barrier, leading to high sustainability in C₂H₄ production. The discovery of this novel up-bottom strategy provides a new approach for optimizing catalytic performance and understanding the synergistic catalytic mechanism of heteroatom sites. The study also investigates the photocatalytic performance and stability of CuAu-DAs-TiO₂, demonstrating superior activity and stability in C₂H₄ production compared to other catalysts. The mechanism of the photocatalytic performance is elucidated through in situ DRIFTS and DFT calculations, revealing the synergistic effect of Cu-SAs and Au-SAs in promoting *CO formation and conversion, as well as suppressing catalyst deactivation under high-*CO conditions.