An all-solid-state electrochromic film was fabricated by laminating tungsten oxide (WO3) and nickel oxide (NiO) thin films deposited by a reactive DC magnetron sputtering on flexible ITO films. The influence of oxygen partial pressure on the crystal structure, microstructure, optical properties, and electrochromic properties of WO3 and NiO thin films were investigated. WO3 and NiO films showed the best electrochromic properties under the flow of Ar:O2=80:20 and Ar:O2=90:10, respectively. The EC film fabricated with an optimized WO3 and NiO films showed a high coloration efficiency, a fast response time, and a stable optical modulation. It is expected that flexible EC window films will pave the way for the next-generation energy-saving windows.