<p>Despite the efficacy of the methylammonium chloride (MACl) additive strategy in stabilizing the α-phase of formamidine-based perovskite materials, a persistent and formidable challenge from the irreversible deprotonation of MA⁺ cation still bothers the fabrication of satisfactory formamidine (FA)-based perovskite photosensitive layers, thereby hindering the performance improvement of resultant perovskite solar cells. To confront this obstacle, numerous methodologies have been proposed and demonstrated their potential, particularly those leveraging additives enriched with carboxylic acid functional groups (-COOH). However, these approaches remain inherently flawed due to the deleterious competitive proton transfer from hydrogen iodide (HI) to the excess -COO^- species, which undermines the integrity of the entire strategy. Herein, an innovative methyl trifluoroacetate (MTFA)-assisted technique is pioneer conducted to control the competitive proton-transfer through its stepwise hydrolysis reaction and the gentle release of -COOH derived from trifluoroacetic acid (TFA) byproduct. By this way, the perovskite precursor solution deprotonation is suppressed and the shelf life of corresponding perovskite precursor solution is greatly prolonged to over 22 days. Concurrently, the robust affinity of -COOCH₃ or byproduct -COOH groups toward uncoordinated Pb²⁺ has also been proven to enable the fine-tuning of perovskite crystallization dynamic, while the residual CF₃COO^- species distributed in the buried interface of perovskite are considered to offer an additional improvement in film carrier behavior via the halogen vacancies passivation. Consequently, high-quality perovskite films with conspicuous crystalline structure, surface morphology and carrier characteristic were obtained, achieving a champion device fabricated from 22 days aging perovskite precursor solution (PPS) with a power conversion efficiency of 26.35% and retaining 94.75% of its initial efficiency after 1464 h of humidity exposure.</p>