Molten salts, good heat transfer and thermal storage media, show potential for various applications, particularly solar thermal power. Nanofluid technology has been applied to molten salts to improve specific heat. In this study, molten salt nanofluids with a base solution of binary molten salt and SiO2 nanoparticles were prepared through high-temperature melting. The specific heat of the molten salt nanofluids was examined with different diameters and mass fractions of nanoparticles, and their optimal diameter and mass fraction were determined. The thermophysical properties of the optimized molten salt nanofluids were experimentally tested and compared with those of pure molten salt. Results reveal that the specific heat of the molten salt nanofluids is higher than that of the pure molten salt. The optimal mass fraction and size of SiO2 nanoparticles at the improved specific heat of the molten salt nanofluids are 1% and 20 nm, respectively. The optimal specific heat of the molten salt nanofluids is 17.8% more than that of the pure molten salt. The density of the optimized molten salt nanofluids is similar to that of the pure molten salt, and the average thermal conductivity is 20.2% more than that of the pure molten salt.