With the fast development of concentrating solar power, a form-stable composite material containing ternary carbonates (K2CO3-Li2CO3-Na2CO3) as high temperature thermal energy storage materials and ceramic MgO as supporting material is proposed to prevent molten salts from leakage and the relevant corrosion. Using cold compression and mixed sintering method, ternary carbonates can be embedded and distributed uniformly in intervals and pores between MgO particles. There aren't chemical reactions between carbonates and MgO during sintering at a high temperature. Among of samples with different mass ratios, the composite material containing 50 wt% ternary carbonates gives the highest mechanical strength. Specific heat capacity and latent heat of the composite material with an optimal ratio of 5:5 can be up to 1.48 J/g K (450-600 degrees C) and 158.7 kJ/kg, respectively. It has a relative low melting point (386.4 degrees C), and it hasn't obvious mass degradation even at 1000 degrees C due to the strong adsorption of carbonates in intervals and pores between MgO particles. Moreover, the composite material shows a good thermal cycling stability after 100 cycles. The resulting form-stable composite material can provide a wider service temperature range and a higher thermal storage density based on the utilization of latent heat and sensible heat of molten salts.