Renewable energy has been actively developed due to the global energy shortage and environmental pollution. The technology of thermal energy storage (TES) is the key to deal with the instability of new energy. Because of its advantages of high heat storage density and wide operating temperature range, molten salts have attracted extensive attention in the medium and high temperature ranges. A form-stable solar salt/steel slag composite phase change material (PCM) was developed in this work for solving the problems of leakage, poor heat transfer performance and high cost of molten salts. The optimum mass ratio of steel slag and molten salt (solar salt) was obtained as 5:5. The microstructure, thermal property and chemical compatibility of the composites were characterized subsequently by scanning electron microscope (SEM), thermogravimetric–differential scanning calorimeter (TG–DSC), laser flash analysis (LFA) and X-ray diffraction (XRD), respectively. The results showed that there was no leakage as the composite material keeps good shape and compact microstructure. The composite material showed good chemical compatibility between molten salt and steel slag. The latent heat was 64.0 kJ/kg, the thermal energy storage density was 945 kJ/kg (100~500℃) and the thermal conductivity was up to 2.23 W/(m?K). Thus, the developed solar salt/steel slag composite PCM is not only of interest to the large-scale application of thermal energy storage, but also provide an excellent option for waste recycling in steel industry.