Molten salt electrolyte for electrolysis should have the properties of low melting point, proper viscosity, density, surface tension, high enough conductivity, low volatility and insoluble metal melt. In order to meet these requirements, a mixture of several salts is usually used. Their melting point is usually lower than that of pure components, but there are many exceptions. Therefore, it is necessary to choose the appropriate mixed salt composition through experiments. Generally, the mixed molten salt of NaCl-KCl-MgCl2 is used to electrolyze magnesium; Na3AlF6-Al2O3 mixed molten salt for electrolytic aluminum. K2TaF7-Ta2O5 mixed molten salt is used for tantalum electrolysis; Bef2, NaF-Baf2 or NaCl-BeCl2 mixed molten salt is used for beryllium electrolysis.