Sodium thiosulfate pentahydrate (Na2S2O3·5(H2O)) has poor cycling stability, experiences phase separation and low crystallization due to large supercooling. In order to improve these properties, the influences of nucleating agent and thickening agent on the supercooling degree, phase separation and exothermic performance of Na2S2O3·5(H2O) were investigated by using the fusion-curing cycle and step-cooling curve system. Observation of the structure and testing of its thermophysical properties by scanning electron microscope, thermostatic bath and differential scanning calorimeter were carried out. The results show that, the addition of sodium pyrophosphate (TSPP) with a mass fraction of 0.08% had a significant improvement effect, and the thickener sodium acrylate (PASS) had the best results. A phase change composite material (CPCM) with a mass fraction of 0.08% TSPP and 2% PAAS had a phase transition temperature of 47.3 °C; the degree of subcooling was 3.6 °C; the phase transition enthalpy was 206.8J/g; the discharge time was 106 min. There was no phase separation during the storage/discharge process. After 100 high and low temperature cycles, the phase transition temperature of the composite was 42.5 °C; the phase transition enthalpy was 202.1J/g, and the cycle stability was good. The improved energy storage material can be used in building heat storage system.