Based on studies of thermal physical properties of ternary carbonates, in order to reduce the melting point of ternary carbonate, ternary carbonates with different components ratio were modified by adding additives including a series of hydroxides or carbonates. And their melting point, initial crystallization point, decomposition temperature, and thermal stability were analyzed by using the Synchronous Thermal Analyzer. The results show that the melting points of mixed carbonates with KOH which can be reduced to around 322℃ to the greatest degree while the melting points of the mixed carbonates with CaCO3 are maintained at approximate 415℃. The initial crystallization temperatures of the mixed molten salts with optimized components ratio are below the 400℃, and their decomposition temperatures are maintained above 800℃. The repeated experiments reveal the good thermal stability of the resultant mixed molten salts with different additives. Combined with analysis of composition, possible modification mechanism of different additive was discussed.