Electrochromic devices have attracted considerable interest for smart windows. However, current development suffers from the requirement of the external power sources and rigid ITO substrate, which not only causes additional energy consumption but also limits their applications in flexible devices. Inspired by galvanic cell, we demonstrate a self-powered flexible electrochromic device by integrating Ag/W<sub>18</sub>O<sub>49</sub> nanowire film with the Al sheet. The Ag nanowire film first acted as the electrode to replace the ITO substrate, then coupled with the Al sheet to induce an open-circuit voltage of ∼0.83 V, which is high enough to drive the coloration of W<sub>18</sub>O<sub>49</sub> nanowires. Remarkably, the flexible self-powered electrochromic device only expends ∼6.8 mg/cm<sup>2</sup> of the Al sheet after 450 electrochromic switching cycles and the size can be easily expanded with an area of 20 × 20 cm<sup>2</sup>, offering significant potential applications for the next generation of flexible electrochromic smart window.