WO3 electrochromic devices (ECDs) with the short response time and high optical contrast show great application potentials in car rearview mirrors and smart windows. With the rapid development of wearable electronics, the fabrication of flexible WO3 films for bendable ECDs has attracted much attention. However, the fabrication of flexible crystalline WO3 films generally requires a sophisticated vacuum vapor deposition technique, including magnetron sputtering and electron-beam deposition due to the high crystallization temperature of WO3 and the limited endurable temperature of polymer substrates. The dense WO3 layers on polymer substrates affect the facile access of electrolyte and fast intercalation/deintercalation of electrolyte ions. In this paper, well-crystallized WO3·H2O microplate powders were pre-synthesized by a chemical reflux method. Monoethanolamine was utilized to intensively dissociate WO3·H2O microplates into a well-dispersed colloidal solution. The flexible nanocrystalline WOx/ polyethylene terephthalate films were fabricated via spin coating deposition and assembled into the flexible ECDs. The accumulated charge repulsion provided by the intense interfacial interaction between amine group and WO3·H2O microplates results in their fast dissociation. The fabricated flexible crystalline WOx electrochromic devices have short coloring and bleaching time of 5.9 s and 5.4 s, respectively, and a coloring efficiency of 146.7 cm2/C, and still exhibit the superior performance after 200 bending cycles.