The high-temperature thermal stability of molten salts determines their upper limit of use. In this paper, two high thermal stability tetragonal molten salts (S1 and S2) were developed independently and the high temperature thermal stability of nitrate molten salts with components ranging from binary to tetragonal was systematically studied. High-temperature cycling experiments and constant-temperature thermal weight loss experiments were conducted on a series of nitrate molten salts in the corresponding operating temperature ranges. The thermal stability of these salts at elevated temperatures was assessed in terms of mass loss, cycling performance, composition, structure, and morphology before and after high-temperature treatment. The results showed that after the high temperature cycling experiment and high temperature heat loss of weight experiment, all the component molten salts showed mass loss, but compared with the binary and ternary molten salts, the developed ones showed lower mass loss and better high temperature thermal stability, and the DSC-TG test indicated that the two tetrameric molten salts developed had a low melting point (~110 ℃) and a high decomposition temperature (~610 ℃).The XRD study indicated that after a long-term high-temperature treatment, the composition of the molten salts remained unchanged, and the relative contents of the components did not change. Meanwhile, the crystallinity and solubility of the mixture increased after high temperature treatment, and the liquid phase composition became more homogeneous. Based on comparative analysis, compared with the binary and ternary mixed salts, the S1 and S2 tetrameric mixed salts have good application value in practical engineering applications.