The static melting method is used to prepare KNO₃–NaNO₃–NaCl ternary mixed molten salt containing 5–20 mass% NaCl, and the thermal physical parameters such as specific heat capacity, melting point, and latent heat of phase change are measured by differential scanning calorimetry. The microstructures of the mixed molten salts are characterized by XRD and SEM. The experimental results show that compared with the base salt, after adding different mass fractions of NaCl, the melting point of the molten salts was reduced by about 24.8%, and the latent heat of phase change was increased by 22.71–52.53 J g⁻¹, the solid and liquid state specific heat capacity of molten salts increased by 3.4% and 2.3% after the addition of 10 mass% NaCl. The addition of 15 mass% NaCl formed a special chlorine salt layer on the surface of the molten salt. This structure reduced the intermolecular voids and enhanced the intermolecular force, which may be the reason for the increase in specific heat capacity and latent heat of phase change.