All-inorganic photoactive CsPbI₃ perovskites easily transform into a photo-inactive non-perovskite phase, but the transition kinetics at the atomic level are currently unknown. In this study, we used first-principle-based stochastic surface walking (SSW) pathway sampling to resolve the phase evolution of the CsPbI₃ transition from γ to δ. The lowest-energy pathway of γ (3D) → Pm (3D) → Cmcm (2D) → Pmcn (1D) → δ (1D) was found to have a transition barrier as low as ∼31 meV/atom. The γ-to-Pm transition was identified as the performance-controlling step. Furthermore, volcano-shaped transition barriers were obtained depending on the ionic size of the substitution dopants, as explained by Pauling’s rule and the cohesive energy. The [010] plane has the largest strain variation during phase transition, implying that the strain involving the [010] axis has a more significant effect on increasing the transition barriers. These results provide rational suggestions and guidance for achieving stable, long-term, all-inorganic halide CsPbI₃ perovskites.