Electrochromic devices (ECDs) can regulate the indoor solar radiation by adjusting optical transmissive properties, showing great commercial potential and important social value of green energy saving. However, the unsafety and high cost of Li⁺ based electrolyte hinder the large-scale and industrialized production of ECDs. Other metal ions have been used as electrolyte ions, but they are rarely reported in all solid state ECDs. In this study, MgF₂ film is used as the solid electrolyte to construct all solid state ECD with the structure of glass/ITO/WO₃/MgF₂/NiO/ITO. The ECD shows the large optical modulation (∼83% at 820 nm, with 100 s durations) and fast response (19.2 s for bleaching and 8.3 s for coloring, with 25 s durations). Moreover, the ECD achieves the extreme transmittance value of colored states Tc ≈ 0%, which can give an absolute private state. This work not only indicates that MgF₂ film can be an alternative to Li⁺ based electrolyte in all solid state ECDs, but also broadens the applications of all solid state EC smart windows to private buildings.