Phase-pure crystallised perovskite is considered an excellent precursor for fabricating high-stability perovskite films with minimal defects. However, currently available protocols for synthesising crystallised perovskites must be conducted in an inert atmosphere or in the presence of an organic solvent as the reaction medium, which hinders mass production. Here, we report the fast synthesis of α-phase-crystallised perovskite powder assisted by moisture in ambient air. Moisture can promote the reaction between PbI2 and organic salts and facilitate complete phase transition, as demonstrated in a joint experimental and theoretical study. Perovskite solar cells with a power conversion efficiency of 24.07 % were achieved using phase-pure crystallised perovskite powder as the precursor. This ambient-air-compatible method opens new vistas to reproducible high-quality precursors for large-scale photovoltaic applications.