Construction of doped-type semiconductor-based photocatalysts can be considered as an excellent platform for enhancing photocatalytic performance owing to their unique properties such as excellent light-absorption ability, more reactive sites and increased photo-induced charge transfer capability. Therefore, we propose a facile hydrothermal synthesis route for the construction of Fe3+ doped Bi2Mo2O9 ultrathin nanosheet with thicknesses of ~4nm. Experimental result indicated that the introduction of Fe3+ into the Bi2Mo2O9 lattice could enhance the light absorption ability and narrow the band-gap of the final product. Photocatalytic results showed that Fe3+ doped Bi2Mo2O9 ultrathin nanosheet exhibited enhanced photocatalytic activity for the decomposition of Rhodamine B (RhB) compared with pure Bi2Mo2O9. Moreover, an enhanced mechanism of photocatalytic performance over Fe3+ doped Bi2Mo2O9 ultrathin nanosheet was investigated and proposed.