Abstract: The saturated vapor pressure of KCl-NaCl-MgCl₂ magnesium electrolyte was measured by constant volume method with the temperature ranging from 650 to 750 degree centigrade. The influence of temperature and component on vapor pressure was researched. The result shows that exponential relationship exists between vapor pressure and temperature, the same as content of magnesium chloride. Vapor pressure increases sharply while temperature is beyond 725 and concentration of MgCl₂ exceeds 15 %. A formula is proposed to describe the relationship of temperature, component and vapor pressure.