High entropy oxides (HEOs), composed of at least five nearly equimolar principal atoms occupying a similar sublattice, demonstrate promising catalytic potential but limited activity. Low-dimensional HEOs, serving as the metastable phase, possess distinctive electronic structures and fully exposed active sites, which anticipate showcasing appealing performance; however, their synthesis remains challenging. Herein, through the incorporation of clusters for kinetic control, a library of single-phase high entropy oxides (HEOs) with single-unit-cell thickness was synthesized under mild conditions (373 K). By modulating the surface entropies, including vibrational, translational, and rotational entropy, the synthesized high entropy subnano-oxides can exhibit structures, such as subnano-wires, subnano-sheets, and spiral coils. Contributed by the fully exposed active sites and electron delocalization among two-dimensional (2D) layer, HEOs presented as subnano-sheets display enhanced catalytic activity for photocatalytic reduction of CO₂ to CH₄, achieving a yield (5777 ± 230.21 μmol·g⁻¹·h⁻¹), which is 41 times higher than that of bulk HEO obtained from the high-temperature calcination synthetic route.