AbstractLong‐term exposure to low concentration indoor VOCs of acetaldehyde (CH₃CHO) is harmful to human health. Thus, a novel isogenous heterojunction CeO₂/Ce‐MOF photocatalyst is synthesized via a one‐step hydrothermal method for the effective elimination of CH₃CHO in this work. This CeO₂/Ce‐MOF photocatalyst performs well in CH₃CHO removal and achieves an apparent quantum efficiency of 7.15% at 420 nm, which presents ≈6.7 and 3.4 times superior to those generated by CeO₂ and Ce‐MOF, respectively. The enhanced efficiency is due to two main aspects including i) an effective photocarrier separation ability and the prolonged reaction lifetime of excitons play crucial roles and ii) the formation of an internal electric field (IEF) is sufficient to overcome the considerable exciton binding energy, and increases the exciton dissociation efficiency by up to 50.4%. Moreover, the reasonable pathways and mechanisms of CH₃CHO degradation are determined by in situ DRIFTS analysis and simulated DFT calculations. Those results demonstrated that S‐scheme heterojunction successfully increases the efficiency of harmful volatile organic compounds elimination, and it offers essential guidance for designing rare earth‐based MOF photocatalysts.