Tungsten oxide nanowire arrays as effective electrochromic working electrodes were fabricated on seed-free FTO glass through a facile solvothermal process. Polyethylene glycol was used as the structure-directing agent to control the crystal growth because of selective absorption on the facet of tungsten oxide crystals. Phase identification and growth mechanism of monoclinic W18O49 nanowire arrays are also extensively discussed. Uniform transparent W18O49 nanowire arrays film exhibits remarkable performance of electrochromic properties, which include high contrast (49.64 % at 632.8 nm), high electrochromic stability (> 3000 CV cycles), and fast coloration/bleaching response (t (c) similar to 7.9 s, t (b) similar to 1.4 s). These properties of the W18O49 nanowire arrays film endow its promising practical applications in large-area smart windows.