FAPbI₃ perovskite solar cells (PSCs) exhibit significant potential for high‐efficiency photovoltaic applications due to their broad light absorption range and exceptional photovoltaic performance. However, their practical implementation faces several challenges, such as the formation of non‐photoactive δ‐FAPbI₃ phases, the poor thermodynamic stability of the α‐FAPbI₃ phase, and intrinsic defects within the films. To address these issues, a multifunctional additive strategy utilizing butylamine acetate (BAAc) was developed to modulate crystallization kinetics and reduce defects in FAPbI₃ perovskites. The ─NH₃⁺ and ─C═O functional groups in BAAc enable bidentate coordination with I⁻ and Pb 2 ⁺ ions, which not only promote the formation of pre‐nucleation clusters and accelerate the crystallization of the α‐phase FAPbI₃ but also passivate intrinsic defects in the FAPbI₃ film. Moreover, BAAc effectively inhibits the formation of the 6H hexagonal intermediate phase, facilitating the preferential crystallization of the photoactive α‐phase FAPbI₃. Additionally, BAAc introduces beneficial compressive strain within the perovskite lattice, thereby enhancing its structural stability. As a result, PSCs incorporating BAAc achieve a power conversion efficiency (PCE) of 26.12% and maintain 90% of their initial PCE after 1000 h of continuous operation under ambient conditions without encapsulation.