The detection of anions in pure water phase with colorimetric sensor is a long standing challenge. As one of the most important anions, F- is associated with nerve gases and the refinement of uranium for nuclear weapons. However, limited by its anions nature, few of the reported colorimetric sensors can successfully applied to detect F-1 in pure water phase. This work designs a colorimetric sensor for F-1 pure water phase detection by taking the advantages of the strong specific binding between F and Si, as well as the color-changing property of H-terminated Si nanowires (SiNWs). The sensor demonstrates ultra-sensitivity, high selectivity, and good stability. The results reveal particular interest for the development of new type aqueous phase anions sensors with SiNWs.