This work investigates the magnetocaloric effect-based green magnetic refrigeration technology, with a focus on Ni-Mn-Ga Heusler alloy as a promising magnetic refrigerant candidate. 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 is adopted. This method enables a detailed analysis of how Mn atoms occupying Ni and Ga sites influence the microstructure, atomic magnetic moments, exchange interactions, and macroscopic magnetocaloric response of the alloy. The results indicate that Mn site occupancy critically affects the magnetic performance: the occupation of Ni sites reduces the total magnetic moment and Curie temperature, thereby reducing the magnetic entropy change; in contrast, Mn occupying Ga sites significantly 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, which significantly exceeds that of the stoichiometric Ni₈Mn₄Ga₄ alloy. Further electronic structure analysis reveals that Mn content variation modulates the density of states near the Fermi level and optimizes orbital hybridization and ferromagnetic exchange interactions, thus adjusting the magnetic phase transition behavior. Critical exponent analysis confirms that the magnetic interactions are inherently long-range and tend toward mean-field behavior with compositional changes. By establishing a clear “composition-structure-magnetism-magnetocaloric performance” relationship on an atomic scale, this work provides theoretical foundations for designing high-performance, low-hysteresis magnetic refrigeration materials.