Abstract: Molten salt phase change thermal storage materials suffer from insufficient thermal conductivity and leakage during the melting process, which limits their practical applications in medium- and high-temperature thermal energy storage. In this study, industrial solid waste fly ash was used as a supporting matrix, and NaCl-KCl eutectic molten salt was selected as the phase change material. NaCl-KCl/fly ash composite phase change thermal storage materials were prepared by a cold-pressing sintering method. The effects of molten salt content on the structural characteristics, thermal storage performance, and thermal cycling stability of the composites were investigated. The results show that NaCl-KCl eutectic molten salt exhibits good chemical compatibility with fly ash, and the molten salt can be effectively encapsulated through the physical confinement effect of the fly ash framework. With increasing molten salt content, the thermal storage performance of the composites gradually improves, while excessive molten salt loading reduces structural stability. Among all samples, the NC4 sample containing 60 wt.% NaCl-KCl exhibits the best comprehensive performance, with a phase change enthalpy of 146.8 J/g, a thermal storage density of 723.03 J/g in the temperature range of 100–700 ℃, and a thermal conductivity of 1.470 W/(m·K). After 50 thermal cycles, the composite material maintains good phase change characteristics and structural stability, demonstrating good thermal cycling stability and potential application prospects for medium- and high-temperature thermal energy storage.