Microcrystalline hexagonal tungsten bronzes (NaxWO3+x/2·yH2O) nanostructures are successfully realized as visible-light-driven photocatalysts via a facile self-assembly route in the presence of sodium ions.Under visible light irradiation (λ>420 nm), correspondingly, these photocatalysts show composition-dependent photocatalytic efficiencies for the degradation of methylene blue.The performance enhanced gradually with increasing the molar ratio ofNa/W (x).We present a combined study of the photocatalysis of the Na interstitially doped h-WO3 by means of optical spectroscopy experiment and density functional theory (DFT) band structure calculations, the excellent agreement between the measured spectra and calculated electronic structure can provide a "band-filling mechanism" for understanding the enhanced photocatalytic activity of microcrystalline hexagonal tungsten bronze.