We have demonstrated that Prussian green (PG) film can be directly grown on fluorine-doped tin oxides (FTO) glass by a facile, template-free and one-step hydrothermal process, and a reaction mechanism for the formation of PG film under the hydrothermal conditions is also proposed. The as-grown PG film exhibits a much better cycling stability than PG film prepared by electrochemical method. The electrochromic device based on the as grown PG film shows reversible and multi-stage color changes among green, blue and colorless states. Large optical modulations of 58% and 42% between colorless and both blue and green state, respectively, fast switching times of 1.1 and 2.4 s and a high coloration efficiency of 79.1 cm(2) C-1 are also achieved for the PG device, making it promising candidates for applications in high-performance smart windows and displays.