AbstractA prerequisite for commercializing perovskite photovoltaics is to develop a swift and eco‐friendly synthesis route, which guarantees the mass production of halide perovskites in the industry. Herein, a green‐solvent‐assisted mechanochemical strategy is developed for fast synthesizing a stoichiometric δ‐phase formamidinium lead iodide (δ‐FAPbI₃) powder, which serves as a high‐purity precursor for perovskite film deposition with low defects. The presynthesized δ‐FAPbI₃ precursor possesses high concentration of micrometer‐sized colloids, which are in favor of preferable crystallization by spontaneous nucleation. The resultant perovskite films own preferred crystal orientations of cubic (100) plane, which is beneficial for superior carrier transport compared to that of the films with isotropic crystal orientations using “mixture of PbI₂ and FAI” as precursors. As a result, high‐performance perovskite solar cells with a maximum power conversion efficiency of 24.2% are obtained. Moreover, the δ‐FAPbI₃ powder shows superior storage stability for more than 10 months in ambient environment (40 ± 10% relative humidity), being conducive to a facile and practical storage for further commercialization.