Micro-nano particle doping is an important method for the modification of molten salt thermal storage materials. By taking a binary carbonate molten salt mixture of 40Li₂CO₃-60Na₂CO₃ (mass fraction) as the base molten salt, CuO and CuCl₂ as the dopants, three composite molten salt phase change thermal storage materials, namely CuO-Li₂CO₃-Na₂CO₃, CuCl₂-Li₂CO₃-Na₂CO₃, and CuO-CuCl₂-Li₂CO₃-Na₂CO₃, were re prepared separately using a high-temperature melting method. Moreover, the thermal properties of these compounds were tested, and the effects of additives on the modification of binary carbonate molten salts and composite molten salt phase change thermal storage materials were investigated. The results show that, the melting point of the Li₂CO₃-Na₂CO₃ molten salt with 0.24% CuO addition decreased by 5.2 ℃, the latent heat of phase change decreased by 98.1 J/g, the average specific heat capacity of the solid phase decreased by 0.39 J/(g·℃), and the average specific heat capacity of the liquid phase decreased by 0.77 J/(g·℃). The upper limit of the operating temperature increased by 4 ℃. For the Li₂CO₃-Na₂CO₃-CuCl₂ molten salt with 0.06% CuO addition, the melting point increased by 9.6 ℃, the latent heat of phase change decreased by 15 J/g, the average specific heat capacity of the solid phase increased by 0.07 J/(g·℃), and the average specific heat capacity of the liquid phase increased by 0.12 J/(g·℃). The upper limit of the operating temperature increased by 17 ℃. Both molten salts exhibited improved thermal conductivity performance after the addition of CuO.