Electrochromic materials with ability of changing color in response to periodically adjusted bias are an important class of optoelectric functional materials. The controllable modulation of light absorption and transmission can make a great contribution in applications such as smart windows, electrochromic displays and antiglare rear-view mirrors. In recent years, electrochromic technology has developed rapidly. However, the research so far mainly focuses on the traditional rigid electrochromic devices (ECD), mostly based on transparent conductive glass such as indium tin oxide (ITO) glass. The rigid electrochromic devices have some noticeable problems such as large thickness, poor conformability, low mechanical strength, high cost, etc., which hinder their further development of electrochromic technology and their forward commercialization. With the upsurge of developing flexible devices that can be used in wearable devices and e-skin, flexible electrochromic devices (FECD) have attracted extensive attention due to their possibility of foldability, wearability and even embeddability, and have become a research hotspot in the electrochromic field. Starting from the materials for preparing FECD, this review systematically summarizes the recent progress and trend of flexible electrochromic devices based on inorganic, organic, inorganic/organic composite and other new materials. The review also focuses on the research progress of up to date stretchable electrochromic devices. At the same time, challenges in performance improvement and practical application of flexible electrochromic devices at the present stage as well as the corresponding measures taken in the literatures are discussed. Finally, the key to the preparation and performance improvement of flexible electrochromic devices is defined, and the future development trend is prospected.