High-entropy ceramics have garnered extensive scientific interest within the realm of advanced optoelectronics, primarily attributed to their exceptional compositional tunability, broad spectral response, and inherent structural stability. Despite rapid progress in high-entropy oxides, systematic theoretical investigations into high-entropy carbonate systems remain significantly limited, hindering the comprehensive understanding of their fundamental physical properties. To bridge the 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 executed via 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 utilized. Structural stability analysis substantiates that the complex MgMnFeCuCO₃ system is thermodynamically robust. Furthermore, Mulliken population and charge density difference analyses reveal 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 as an antiferromagnetic semiconductor featuring a theoretical direct bandgap of 3.32 eV. A pivotal finding of this research is the decisive role of the crystal field and Jahn-Teller distortion within the FeO₆ and CuO₆ octahedra. These structural distortions lift the orbital degeneracy of the Fe-3d and Cu-3d states, successfully introducing discrete impurity levels within the bandgap. Specifically, Cu-3d states form shallow levels near the valence band maximum, whereas Fe-3d states manifest as sharp, localized deep-level resonance peaks. Optically, these intermediate states facilitate electron transitions, acting as effective bridges that substantially augment the material's visible-light absorption capacity. The compound yields a static dielectric constant of 4.158. The primary optical absorption is driven by transitions from the O-2p orbitals at the valence band edge to the Mg-2s and C-2p states at the conduction band bottom. 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 displays anomalous dispersion and a distinct energy loss function peak near 420 nm, indicative of collective electronic oscillations. Ultimately, this research elucidates the microscopic mechanisms by which transition metal orbital splitting modulates band structures in high-entropy systems. These findings provide valuable insights into their fundamental properties and may contribute to the rational design of high-entropy materials for targeted optoelectronic applications.