The phase transformation during the aging process of magnesium alloys is crucial for controlling the precipitation-strengthening phase. This study discovered a novel phase transition from γ' to γ'' with experiments and utilized a combination method of high-throughput density functional theory (DFT) and kinetic Monte Carlo (KMC) methods (HKD) to calculate and analyze the γ' → γ'' transition in Mg-Gd-Zn-Mn alloys from both thermodynamic and statistical perspectives. The results indicate that during the doping process of 13 representative elements, Mn doping is relatively stable. When Mn is doped into the γ' precipitate, it preferentially diffuses to the center of the structure, specifically to one of the bridge sites. This diffusion repels Gd atoms, keeping them away from the middle atomic layer. Consequently, the Zn and Mn atomic networks jointly form a coordinated atomic plane, ultimately completing the transformation to the γ'' precipitate.