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The main reason for large energy loss in all‐inorganic perovskites is ascribed to the slow nucleation and fast crystallization of all‐inorganic perovskite films. Herein, a manipulating strategy is demonstrated to simultaneously realize rapid nucleation and slow crystal growth of CsPbI3 perovskite films by employing solvent molecular sieves in the antisolvent. First, the antisolvent treatment of mixed chlorobenzene and ethyl alcohol can induce the instantaneous supersaturation of perovskites to achieve rapid nucleation. Subsequently, the molecular layer of phthalimide (2‐N) molecules on the perovskite surface can be used as solvent molecular sieves to precisely control the evaporation of the solvent through molecule–solvent interactions. In addition, the molecules remaining on the surface can also effectively passivate the surface defects and improve the device performance. By this strategy, a synchronous regulation of rapid nucleation and slow crystal growth of perovskite films is realized for the first time. As a result, the CsPbI3 film with 2‐N treatment presents high‐quality crystallinity with large grains and less defects. The champion device exhibits an outdoor power conversion efficiency (PCE) up to 20.14% under AM1.5G illumination, and an indoor PCE up to 40.07% (Pout:133.9 µW cm–2) under a commonly used light‐emitting diode light source (2956 K, 1062 lux). [ABSTRACT FROM AUTHOR]