This study investigates the magnetocaloric effect-based green magnetic refrigeration technology, with a focus on Ni-Mn-Ga Heusler alloys as promising magnetic refrigerant candidates. To elucidate the role of Mn-rich composition in regulating the magnetic and magnetocaloric properties, a multi-scale computational approach integrating first-principles calculations and Monte Carlo simulations was adopted. This methodology enables a detailed analysis of how Mn atoms occupying Ni versus Ga sites influence the alloy’s microstructure, atomic magnetic moments, exchange interactions, and macroscopic magnetocaloric response. The results demonstrate that Mn site occupancy critically governs the magnetic performance: occupation of Ni sites reduces the total magnetic moment and Curie temperature, thereby diminishing the magnetic entropy change; in contrast, Mn occupying Ga sites markedly enhances both the total magnetic moment and the magnetic entropy change. Notably, the Ni8Mn7Ga1 alloy achieves a maximum magnetic entropy change of 2.32 J·kg^-1·K^-1 under a 2 T magnetic field, substantially surpassing that of the stoichiometric Ni8Mn4Ga4 alloy. Further electronic structure analysis reveals that Mn content variation modulates the density of states near the Fermi level, optimizes orbital hybridization and ferromagnetic exchange interactions, and consequently tailors the magnetic phase transition behavior. Critical exponent analysis confirms that the magnetic interactions are long-range in nature and tend toward mean-field behavior with compositional changes. By establishing a clear “composition-structure-magnetism-magnetocaloric performance” relationship at the atomic scale, this work provides theoretical foundations for designing high-performance, low-hysteresis magnetic refrigeration materials.