Molten salt is an important heat storage and heat transfer medium in solar thermal power generation technology due to its high heat capacity, wide working temperature range, and low cost. Adding nanoparticles (usually by the two-step method with ultrasonic dispersion) can increase the specific heat of molten salt. Thus, the molten salt nanofluids can increase the heat capacity and decrease the heat storage cost of a solar thermal power generation system. However, nanoparticles are easy to agglomerate in the molten salt; moreover, after agglomeration, the performance of molten salt nanofluids is degraded. A two-step method with high-temperature melting in preparing molten salt nanofluids is proposed in this paper. Molten salt nanofluids were prepared by high-temperature melting. The base solution was a low-melting point molten salt and the nanoparticles were SiO2 with a diameter of 20 nm. The specific heat was measured and the nanoparticle dispersity was analyzed. The stability of the molten salt nanofluids was studied, and the results were compared with those prepared by ultrasonic dispersion. The average specific heat of molten salt nanofluids prepared by high-temperature melting was 1.789 J/(g.K), which was close to that of molten salt nanofluids prepared by ultrasonic dispersion and 16.4% higher than that of pure molten salt. The molten salt nanofluids prepared by ultrasonic dispersion showed poor thermal stability under high-temperature conditions, and the average specific heat decreased by 8.5% after only 200 h. The thermal stability of molten salt nanofluids prepared by high-temperature melting showed a highly stable performance in long-time experiments. The variation of specific heat was less than 5% after 2000 h under the same high-temperature experimental condition. The molten salt nanofluids obtained by high-temperature melting showed better stability and long performance than those obtained by ultrasonic dispersion. Therefore, the two-step method with high-temperature melting is stable and reliable for preparing molten salt nanofluids.