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Formaldehyde (HCHO), as one of the main indoor toxic pollutions, presents a great threat to human health. Hence, it is imperative to efficiently remove HCHO and create a good indoor living environment for people. Herein, a layered perovskite material SrBi₂Ta₂O₉ (SBT), was studied for the first time and exhibited superior photocatalytic efficiency and stability compared to commercial TiO₂ (P25). Furthermore, a unique dark–light tandem catalytic mechanism was constructed. In the dark reaction stage, HCHO (Lewis base) site was adsorbed on the terminal (Bi₂O₂)²⁺ layer (Lewis acid) site of SBT in the form of Lewis acid-base complexation and was gradually oxidized to CO₃²⁻ intermediate (HCHO → DOM (dioxymethylene) → HCOO⁻ → CO₃²⁻). Then, in the light reaction stage, CO₃²⁻ was completely converted into CO₂ and H₂O (CO₃²⁻ → CO₂). Our study contributes to a thorough comprehension of the photocatalytic oxidation of HCHO and points out its potential for day–night continuous work applications in a natural environment.