Inspired by hydrothermal synthesis of inorganic crystallites, we apply one-step hydrothermal polymerization to directly grow polyaniline on fluorine-doped tin oxide (FTO) substrate. In contrast to classical procedure that requires long reaction times and low temperature, hydrothermal polymerization allows for only 3 h under hydrothermal condition. Investigation of the morphology via SEM suggests that PANI fiber aggregates uniformly on FTO substrate. The PANI film was used as electrochromic layer in an electrochromic device and the obtained PANI film shows high electrochromic performance. We believe that this study provided a novel way to directly grow PANI on FTO substrate and, thus, has some impact on the applications of other conjugated polymers in electrochromic and electrochemical devices.