Provided is a method for producing hexagonal tungsten oxide, the method including preparing an alkaline solvent having a pH of 8 to 9, which contains at least one of water or alcohol, adding tungsten chloride to the alkaline solvent to form a first reaction solution, adding an additive to the first reaction solution to form a second reaction solution, and adding strong acid to the second reaction solution to form nanoparticles. The additive includes any one of an amine compound having 1 to 8 carbon atoms or an aliphatic hydrocarbon derivative having 10 or more carbon atoms.