Abstract: The supercooling and phase separation of hydrated salt phase change heat storage materials are two key issues which affect its thermal stability and thermal performance. MgCl₂∙6H₂O (MCH) and MgSO₄∙7H₂O (MSH) are used as the research materials, which are the phase change heat storage materials at moderate and low tempera-tures, and activated carbon (ACC) is used as the additive, The MCH-MSH-ACC composite phase change system was prepared by the melt blending method, then, the MCH-MSH-ACC/PA composite phase change system is prepared with paraffin (PA) as the modifier. The effects of PA on the phase change enthalpy, phase change temperature, subcooling degree and phase separation phenomenon of the composite phase change materials were studied. The re-sults show that the addition of trace PA is beneficial to improve the phase change heat storage performance of the MSH-MCH-ACC/PA system. Compared with other PA content systems, the system with 0.5wt% PA perform the shortest time in the heat storage stage and the maximum heat release time in the exothermic phase, whose initial phase change enthalpy value reach 321.75 kJ/kg, and the stable enthalpy is 310.25 kJ/kg after the cycle tests. The prepared new blend MSH-MCH-3wt%ACC/0.5wt%PA composite material has good heat storage property and good thermal cycle stability.