This study employs empirical force fields and MD simulations to elucidate how water molecules assist the B↔Y phase transitions in inorganic MHPs at the atomistic level. By employing an efficient rigid ion model capable of accurately capturing the physics of the MHP-water system, we are able to model the microsecond-scale dynamics of 16 nm-long CsPbI 3 nanowires in a humid environment. We select the typical inorganic MHP CsPbI 3 as the model system, but the conclusions drawn are equally applicable to similar perovskite systems. Through analysis of the nucleation barriers, we conclude that the B↔Y phase transition in perovskite materials involves a two-step crystal-amorphous-crystal (C-A-C) mechanism. Furthermore, through analysis of the phase transition barriers, interface ion diffusion barriers, and simulation trajectories in systems with different water contents, we find that water molecules enhance the diffusion rate of interface ions by binding with interface cations and releasing anions, thereby lowering the phase transition barrier and accelerating the transition process.