A method for synthesizing manganese niobate MnNb2O6 nanosheet belongs to the field of functional materials. The manganese niobate MnNb2O6 nanosheet may be prepared by: using an ethanol solution of manganese chloride (MnCl2.4H2O) and surfactant mixture and thermally treating amorphous phase niobium pentoxide with ethanol solvent; a surfactant used herein is an organic compound comprising sulfonic groups. The obtained nanosheet has a good application prospect in the field of treatment of hexavalent chromium ion wastewater.