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Tungsten oxide (WOx) electrochromic (EC) devices are capable of reversibly displaying blue colour and becoming colourless upon charge injection and extraction induced by the applied voltage, which makes them appropriate for various applications including EC displays, smart windows, sensors, smart supercapacitors and energy storages.Here, the controlled synthesis of W18O49 thin films made of nanowires were grown directly on fluorine-doped tin oxide glass substrates via a simple hydrothermal method.The structure and properties of W18O49 thin films were investigated by scanning electron microscope (SEM), X-Ray Diffraction (XRD), optical transmittance spectra, and cyclic voltammetry (CV).SEM analysis revealed that the diameter of the nanowires of final products was significantly influenced by adding oxalic acid and citric acid.The diffraction peaks of XRD patterns could be clearly indexed as monoclinic W18O49.Adding two organic acids had no obvious influence to crystal quality of products.The films exhibited tunable transmittance modulation under different voltages, and adding organic acids showed high color contrast ratios.The present electrochromic device composed of the W18O49 thin films had high electrochromic stability, color contrast and reasonable switching response.We believe that our success in synthesis of W18O49 nanowires thin films may open the door to important opportunities in the development of electrochromic devices.