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Enhancing crystallinity, passivating the grain boundary and interfacial defects have been validated to be critical for improving the power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Herein, a synergetic co‐modulation and co‐passivation strategy is proposed to simultaneously enhance crystallinity and passivate the grain boundary and surface defects of FAPbI3 based PSCs. The 4‐fluoro‐phenethylammonium iodide (4‐F‐PEAI) added in precursor solution and poly (9‐vinylcarbazole) (PVK) added in antisolvent can jointly modulate the crystallization of FAPbI3 films. The 4‐F‐PEAI‐derived 2D perovskite, which is spontaneously formed at the grain boundaries of FAPbI3, can passivate the defects effectively. In the meantime, PVK left on top of a FAPbI3 layer can passivate the surface defects and meanwhile function as an interfacial barrier layer between FAPbI3 and hole transport layer (HTL) to mitigate the detrimental interfacial charge recombination. With the holistic benefit of the enhanced crystallinity, reduced defects and trap sites, and mitigated non‐radiative recombination and suppressed ion migration, the encouraging PCEs up to 21.6% is achieved for the resulting modified PSCs. Additionally, this strategy endows the device with notably enhanced operational stability under continuous exposure to illumination, with more than 84% of the initial PCE being maintained after continuous illumination for 800 h.