With the development of the miniaturization of electronic equipment and lightweight weapon equipment, there are new requirements for electromagnetic wave absorption material (EMWAM). EMWAM has outstanding electromagnetic wave absorption properties and lightweight characteristics become an important direction of research. In this study, graphene/g-C₃N₄ (GGCN) EMWAM was first synthesized in situ by simple heat treatment, in which the g-C₃N₄ had a porous structure and dispersed on the surface of graphene. The impedance matching of the GGCN was well adjusted by decreasing the dielectric constant and attenuation constant due to the g-C₃N₄ semiconductor property and the graphite-like structure. The EMW loss mechanism of GGCN was also analyzed by simulating GGCN’s electric field mode distribution and resistance loss power density. The analysis result shows that the distribution of g-C₃N₄ among GGCN sheets can produce more polarization effects and relaxation effects by increasing the lamellar spacing. Furthermore, the polarization loss of GGCN could be increased successfully by porous g-C₃N₄. Ultimately, the EMW absorption property of GGCN is optimized significantly, and GGCN exhibits excellent EMW absorption performance. When the thickness is 2 mm, the effective absorption bandwidth (EAB) can reach 4.6 GHz, and when the thickness is 4.5 mm, the minimum reflection loss (RL_min) at 4.56 GHz can reach −34.69 dB. Moreover, the practical application of EMWAM was studied by radar cross-section (RCS) simulation, showing that GGCN has a good application prospect.