The preparation of highly absorbing and high-performance electromagnetic wave-absorbing materials remains a significant challenge. In this paper, carbon matrix composites modified with sodium inorganic salts (CMS) have been successfully prepared by using the solvent evaporation method in combination with the hydrothermal treatment and high temperature carbonization processes. The composition of the product can be manipulated by varying the material ratios, which in turn affects the interface and dielectric properties of the material. Sodium carbonate (Na2CO3) was produced as a transition phase between sodium silicate and carbon matrix due to high temperature. According to differential charge density a significant charge accumulation found between the interfacial of Na2CO3 and carbon layer is helpful to polarization. The synergistic effect of polarization and conduction loss due to the formation of multiple heterogeneous interfaces and carbon matrix results in excellent electromagnetic wave absorption performance. The minimum reflection loss (RLmin) is −55.49 dB when the thickness is 3.1 mm. Furthermore, CMS composites exhibit notable flame retardancy and thermal insulation properties, rendering them suitable for a diverse range of applications. These remarkable achievements provide new ideas for the design and development of efficient electromagnetic wave absorption materials.