Halide perovskites have shown superior potentials in flexible photovoltaics due to their soft and high power‐to‐weight nature. However, interfacial residual stress and lattice mismatch due to the large deformation of flexible substrates have greatly limited the performance of flexible perovskite solar cells (F‐PSCs). Here, ammonium formate (HCOONH₄) is used as a pre‐buried additive in electron transport layer (ETL) to realize a bottom‐up infiltration process for an in situ, integral modification of ETL, perovskite layer, and their interface. The HCOONH₄ treatment leads to an enhanced electron extraction in ETL, relaxed residual strain and micro‐strain in perovskite film, along with reduced defect densities within these layers. As a result, a top power conversion efficiency of 22.37% and a certified 21.9% on F‐PSCs are achieved, representing the highest performance reported so far. This work links the critical connection between multilayer mechanics/defect profiles of ETL‐perovskite structure and device performance, thus providing meaningful scientific direction to further narrowing the efficiency gap between F‐PSCs and rigid‐substrate counterparts.