A flexible transparent electrochromic device, which includes the following components, each of which is a flexible film: a working electrode comprising a transparent conducting substrate supporting an working electrode active material; a counter electrode including a transparent conducting substrate supporting a counter electrode active material; a solid polymer electrolyte (SPE) including a solution of a lithium salt in a polymer solvent. A method for preparing an electrochromic device includes the steps of: preparing a working electrode film, preparing a counter electrode film, preparing a polymer electrolyte film, and assembling the electrodes and the electrolyte, the method being implemented continuously.