Energy-storage technology using phase-change heat-storage materials (PCHMs) has become a research focus to address the mismatch between the supply and demand of solar energy photothermal utilization. Ammonium aluminum sulfate dodecahydrate (AASD) is a promising PCHM due to its suitable phase-change temperature and high latent heat of fusion. However, there are disadvantages of this material, such as undercooling and a short service life. In this study, AASD was mixed with stearic acid, CaF2, and deionized water to prepare a composite PCHM. The results showed that the degree of undercooling decreased by 10 K with the optimized weight composition ratio of 92.2:5:1.8:1 (AASD:stearic acid:CaF2:deionized water). After repeated melting and solidification, the attenuation ratio decreased from 44.3% to 17.6%. The microstructure and heat-transfer characteristics of the PCHM were simulated and analyzed using the double temperature and equivalent specific heat capacity methods. The heat-transfer characteristics of the PCHM were quantified. The differential equation for the three-dimensional, unsteady heat conduction of the PCHM at the solid-liquid interface during the phase-change process was obtained, and the numerical results were in good agreement with the experimental results. Highlights For the first time, the organic compound, stearic acid, was combined with the inorganic hydrated salt, AASD. A new ammonium aluminum sulfate dodecahydrate/stearic acid composite phase-change heat-storage material was prepared. Superior heat-transfer characteristics were obtained when the ratio of AASD, stearic acid, CaF2, and deionized water was 92.2:5:1.8:1, and the degree of undercooling decreased by approximately 10 K. With the addition of stearic acid, the latent heat of fusion of the PCHM could be well maintained after repeated melting and solidification. The attenuation ratio of PCHM after 200 cycles was significantly lower than that of AASD, and the service life doubled. The differential equation for the three-dimensional, unsteady heat conduction of the PCHM at the solid-liquid interface during the phase-change process was obtained.