Abstract: Using first-principles calculations based on density functional theory and employing the Vienna Ab-initio Simulation Package (VASP) quantum mechanics calculation software, the formation energies, equilibrium lattice constants, and tetragonal distortion curves of Ni₂Mn_1.5In_0.5 alloy were systematically investigated under different vacancy defect structures. The optimal occupancy of defects within the vacancy defects was determined, and the law that the equilibrium lattice constants are influenced by both atomic radius and magnetism was verified. By calculating the tetragonal distortion curves of the austenite phase of the alloy under various vacancy defects, the influence law of point defects on the martensitic phase transformation behavior of the alloy was explored and the defect configurations that induce martensitic phase transformation were predicted. The results indicate that alloys containing V-Ni and V-Mn1-1 vacancy defects undergo martensitic phase transformation, while those with other vacancy defect configurations do not. Vacancy defects preferentially occupy the sublattice sites of Ni atoms with smaller atomic radii, leading to a decrease in the stability of the austenite phase, with stability gradually diminishing as the atomic radius of the substituted atoms increases. Furthermore, from the perspective of electronic density of states, the mechanism underlying the change in austenite phase stability was revealed. It was found that in the V-Ni point defect with the FIM (ferrimagnetic) austenite phase, the total density of states (DOS) for spin-up and spin-down electrons exhibits poor symmetry, and the total DOS near the Fermi level is at a peak position, indicating a relatively large total magnetic moment of the alloy and poor structural stability of the austenite phase.