Cold compression and mixed sintering methods have attracted attention for their applicability to industrial mass production. Due to the wettability of expanded graphite (EG) and salt chlorides, the decoupling of the encapsulation effect of EG and ceramics should be considered in the form-stable composite materials ratio of ternary chlorides (TC)/MgO ceramics/EG. In this study, 40 wt%-55 wt% MgO and 5 wt%-20 wt% EG are added to the composites, respectively. It is found that the composite system can be loaded with nearly 45 wt% molten salts. The 45 wt% TC-15 wt% EG-40 wt% MgO sample named 15EG-40MgO is considered a close to optimal formulation in terms of thermal conductivity enhancement and energy density reduction. This proportional composite has a thermal conductivity of 8.86 W∙m⁻¹∙K⁻¹, and in the temperature range of 300–600 °C, the thermal storage density reaches 439.53 J·g⁻¹. With the increase of TC content, the density of the composites shows a decreasing trend. However, all samples have excellent mechanical strength due to the MgO ceramics as the supporting material. Moreover, these composites can be easily integrated into a solar thermal storage system, which proposes a new strategy for applying a high-temperature packed-bed containing salt-based composite phase change materials.