High-entropy ceramics have garnered extensive scientific interest in the realm of advanced optoelectronics, owing to their exceptional compositional tunability, broad spectral response, and inherent structural stability. Although rapid progress has been made in high-entropy oxides, systematic theoretical investigations into high-entropy carbonate systems remain significantly limited, thereby hindering a comprehensive understanding of their fundamental physical properties. To bridge this knowledge gap, this study systematically elucidates the electronic structure, chemical bonding characteristics, and optical responses of a novel high-entropy carbonate, MgMnFeCuCO₃. First-principles calculations were performed using the CASTEP module utilizing the generalized gradient approximation (GGA-PBE). To accurately describe the strong on-site Coulomb interactions inherent to the localized 3d electrons of the transition metals, the LDA+U method (with U = 3 eV for Mn, Fe, and Cu) was adopted. Structural stability analysis substantiates that the complex MgMnFeCuCO₃ system is thermodynamically robust. Furthermore, Mulliken population and charge density difference analyses reveal the internal bonding network; this network is defined by C—O covalent interactions within the carbonate subunits and predominantly ionic metal-oxygen (M—O) bonds. Accordingly, significant charge transfer occurs from the metallic cations to the oxygen anions, facilitating intense p-d orbital hybridization, with manganese exhibiting the maximal degree of ionicity and valence electron delocalization. Electronic structure calculations demonstrate that MgMnFeCuCO₃ is an antiferromagnetic semiconductor featuring a theoretical direct bandgap of 3.32 eV. Notably, the crystal field and Jahn-Teller distortion within the FeO₆ and CuO₆ octahedra play a critical role. These structural distortions lift the orbital degeneracy of the Fe-3d and Cu-3d states, thereby giving rise to discrete impurity levels within the bandgap. Specifically, Cu-3d states form shallow levels near the valence band maximum, whereas Fe-3d states appear as deep-level resonance states, localized deep-level resonance peaks. These intermediate states facilitate electron transitions, thereby significantly enhancing the material's visible-light absorption capacity. The compound exhibits a static dielectric constant of 4.158. The primary optical absorption is driven by transitions from the O-2p orbitals at the valence band maximum (VBM) to the Mg-2s and C-2p states at the conduction band minimum (CBM). This manifests as a prominent visible-spectrum peak, reaching a maximal absorption coefficient of 4.12×10⁴ cm^–1 at approximately 490 nm. Moreover, the material exhibits anomalous dispersion and a distinct peak in the energy loss function near 420 nm, indicative of plasmon resonance. Ultimately, this research elucidates the microscopic mechanisms by which the transition metal orbital splitting modulates the band structures in high-entropy systems. These findings provide valuable insights into their fundamental properties and may offer a basis for the rational design of high-entropy materials for tailored optoelectronic applications.