Low-cost and facile approach to grow quasi-one-dimensional hexagonal tungsten trioxide nanobundles (Q1D hex-WO3 NBs) on indium tin oxide (ITO) coated flexible polyethylene terephthalate (PET) substrate via hydrothermal method combined with electrophoretic deposition (EPD) technique is reported. Influence of the precursor concentration and hydrothermal reaction time on the morphology and size of as-synthesized WO3 nanostructures are investigated. Well-designed nanostructure exhibits remarkable enhancement in the electrochromic (EC) properties. In particular, a significant optical modulation (44% at 700 nm), fast color switching speed (bleaching: 22 s and coloration: 9 s), high coloration efficiency (89.8 cm2 C-1 at 700 nm), high Li+ diffusion coefficient (8.92 × 10-10 cm2 s-1), and excellent cycling stability (74% after 1000 cycles) are achieved for the Q1D hex-WO3 NBs film. Improved EC properties are mainly attributed from the Q1D hex-WO3 NBs. Li+ can transport quickly and easily diffused into the storage sites of the interface of the NBs. Furthermore, we also reported a large areal flexible EC device (70 mm × 70 mm) with a simple two-electrode configuration exhibiting outstanding mechanical flexibility and high optical contrast. The Q1D hex-WO3 NBs film acts as an excellent EC material.