As a major indoor volatile organic compound, acetaldehyde is considered to be toxic when applied externally for prolonged periods; it is an irritant and a probable carcinogen. In this work, Bi2WO6/BiOI/allophane (BW/BI/A) composites with different molar ratios of Bi2WO6:BiOI were prepared by either mechanical mixing or a hydrothermal synthesis. The adsorption capacity and photocatalytic activity of the prepared composites were evaluated for the adsorption and photodegradation of gaseous acetaldehyde in the dark and under visible light irradiation, respectively. SEM revealed that with increasing BiOI content, the overall morphology of the BW/BI/A composite was altered because BiOI nanoparticles gradually occupied the surfaces of the nanosheets, which formed flower-like structures, and eventually covered the surfaces of the Bi2WO6 particles. Adsorption affinities and preferential adsorption sites of acetaldehyde molecules on the Bi2WO6, BiOI, and allophane components of the BW/BI/A composite were also predicted using molecular dynamics simulations. The BW/0.5BI/A composite exhibited high adsorption capacity, excellent photocatalytic performance and good stability owing to its large specific surface area, greater number of easily accessible active sites, facilitated diffusion of reactants, multiple scattering of incident light, and formed p–n heterojunction, which suggest that it can be used in environmental remediation in the future.