Abstract: Indoor formaldehyde pollution poses long-term hazards to human health. Photocatalytic oxidation is considered an ideal approach for gaseous formaldehyde removal due to its environmental friendliness and sustainability. However, because indoor gaseous formaldehyde molecules have low concentration and high diffusivity, the concentration of reactants at the interface is limited in traditional gas-solid photocatalytic systems, severely restricting the actual formaldehyde removal efficiency. To address this issue, this study utilizes the self-hygroscopic properties of H₅IO₆ to construct an in-situ 'gas-liquid-solid' three-phase reaction interface on the surface of BiVO₄ achieving enrichment of gaseous formaldehyde and efficient generation of hydroxyl radicals through a surface hydration layer. Subsequently, the BiVO₄@H₅IO₆ catalyst is loaded onto the surface of wood to prepare a functional coating. The results demonstrate that H₅IO₆ forms a uniform and dense coating layer on BiVO₄, significantly increasing the surface hydroxyl groups and adsorbed water content, while inducing interfacial electronic structure modulation to promote the separation and migration of photogenerated charge carriers. Photocatalytic performance tests reveal that the BiVO₄@H₅IO₆ composite achieves a formaldehyde adsorption efficiency of 37.4% in the dark stage. Upon 3 h of light irradiation, the degradation efficiency reaches 68.2%, which is markedly higher than that of pristine BiVO₄ (45.2%), indicating the crucial role of the interfacial hydrated layer in enhancing photocatalytic activity. Furthermore, when applied as a wood surface coating, the resulting layer exhibits a thickness of approximately 4.2 μm and an adhesion grade of 2, maintaining good structural stability after 15 days under simulated indoor conditions. In summary, this work proposes a self-hygroscopic interface engineering strategy to construct a gas-liquid-solid triphase system, providing new insights into the design of wood-based functional coatings for indoor air purification applications.