Abstract: A low-melting-point molten salt composed of NaCl, CaCl₂, and MgCl₂ was developed via thermodynamic calculations and experimental validation. A set of thermodynamic database for the NaCl–CaCl₂–MgCl₂ molten salt was built. The molten salt composition was optimised based on the phase diagram calculations, and the melting point measured in the experiment was 418.5°C. The molten salt was purified by adding excess Mg, resulting in a final Mg content of 738.0 μg/g. Compared with the NaCl–CaCl₂–MgCl₂ salt, the Mg-containing salt exhibited enhanced thermal stability, with a reduced weight loss of only 5.0wt% at 728.0°C. The average specific heat capacities of the solid and liquid phases were measured at 0.89 and 1.44 J/(g·°C), respectively, indicating improved heat retention. Notably, the differences in the melting temperature and fusion enthalpy remained negligible. For the first time, the ultrafast dynamics of this ternary molten salt were characterized using polarization-selective mid-infrared pump-probe spectroscopy. The Mg²⁺ vibrational dynamics (4.5 ps) were slower than those of the water peak (2.3 ps), whereas the behavior of the water peak closely resembled that in pure water. Collectively, this study enriches the physicochemical database of molten salt and offers theoretical and technical guidance for salt purification, with significant implications for the comprehensive utilization of salt-lake resources with abundant reserves in China.