Introducing high-index facet into crystals is a promising way to provide additional catalytically active sites for boosting intrinsic activity. This work presents a simple one-pot annealing way to fabricate highly photoactive pristine polyhedral α-Fe₂O₃ single crystal, using melamine as solvent and coordination agent. The HR-TEM results realized that the as-prepared sample was enclosed by high-index {116} facet, which was always not presented in traditional hydrothermal method. The photoelectrochemical (PEC) measurements have been carried out and showed that the polyhedron samples exhibited significantly enhancement in the photocurrent density, which is nearly 28 times higher than that of the α-Fe₂O₃ prepared without melamine under the same conditions. Our work may guide future designs for high-index crystal facet exposed of metal oxides and suggest paths for further improvements to allow technological development and use.