It is well-known that tungsten oxide (WO3) is extensively used for electrochromic (EC) devices due to its special properties, such as high coloration/bleaching response time, good coloration efficiency, etc. The preparation of flexible EC devices is highly desirable for various applications. However, the fabrication of flexible EC devices with improved performance is still a major task due to their limited experimental protocols. In this study, we have prepared highly flexible transparent films consisting of WO3 and silver nanowires coated on nanofiber-reinforced composite films (WO3/AgNW-NCF). A novel flexible transparent WO3/AgNW-NCF is used for EC applications. The as-fabricated EC device based on WO3/AgNW-NCF demonstrated good coloration efficiency (161.3 cm2 C−1), excellent stability (81.5%) and rapid response time for coloration (2.9 s) and bleaching (3.2 s). The fabricated transparent flexible EC device may potentially be useful for designing an outstanding EC device for smart window applications.