Abstract: Sodium thiosulfate pentahydrate(Na₂S₂O₃·5H₂O) phase change energy storage material has high phase change latent heat, energy storage density, etc. However, Na₂S₂O₃·5H₂O has undercooling and phase separation, the performance is unstable. Adding the nucleating agent K₂SO₄, CaSO₄·2H₂O, Na₄P₂O₇·10H₂O and enhance heat transfer material expanded graphite(EG) could improve the performance, EG/Na₂S₂O₃·5H₂O phase change energy storage composite with stable properties was prepared. Test results show that the nucleation performance of K₂SO₄ for Na₂S₂O₃·5H₂O is worse, both CaSO₄·2H₂O and Na₄P₂O₇·10H₂O have better nucleation performance, which could significantly reduce the undercooling degree of Na₂S₂O₃·5H₂O. However, the CaSO₄·2H₂O system is unstable, adding the mass fraction 3.0% of Na₄P₂O₇·10H₂O could reduce the undercooling degree of Na₂S₂O₃·5H₂O to within 1℃, adding mass fraction 0.5%-2.0% of EG could reduce the undercooling degree to about 2℃, and mass fraction 7% of EG can completely eliminate phase separation of the system. Therefore, the optimal composition for phase change materials is 7.0% of EG, 3.0% of Na₄P₂O₇·10H₂O, and 90.0% of Na₂S₂O₃·5H₂O. The latent heat of EG/Na₄P₂O₇·10H₂O/Na₂S₂O₃·5H₂O composite is 192.5 kJ·kg^-1. No liquid leakage and no phase separation when the phase transition. The thermal storage time is shorter 22.3% than pure substance, and heat release faster.