Abstract: To meet the requirement of high temperature of solar heat transfer and thermal storage,a new type of Na<SUB>2</SUB>CO<SUB>3</SUB>-K<SUB>2</SUB>CO<SUB>3</SUB> molten salt was prepared by static fusion method according to the phase diagram data of carbonate.It was modified by high melting point additives,such as NaCl,KCl,and Li<SUB>2</SUB>CO<SUB>3</SUB>.Thermal properities such as the melting point,phase transition latent heat,ion change,and thermal stability of the mixed molten salts were characterized by thermal gravimetric and differential thermal analysis,gravimetric method,potentiometric titration method,and thermal circulation method.Experimental results showed that the modified carbonate molten salt with Li<SUB>2</SUB>CO<SUB>3</SUB> had better melting point and phase transition latent heat data than that with NaCl and KCl,but the cost was high.Therefore,it was not suitable to be the solar heat transfer medium.The carbonate molten salt modified by low-cost NaCl is a perfect high-temperature carrier,whose melting point was 133 ℃ lower than that of binary carbonate salt,phase transition latent heat was 1.9 times as large as binary carbonate salt and thermal stability was good under the temperature of&nbsp; 850 ℃.