AbstractFormamidinium (FA)‐based lead triiodide have emerged as promising light‐harvesting materials for solar cells due to their intriguing optoelectronic properties. However, obstacles to commercialization remain regarding the primary intrinsic materials instability, wherein volatile organic components of FA⁺ cations are prone to escape under operational stressors. Herein, stabilizing FA‐based perovskite through toughening the interface with the symmetric molecule of 1,1′‐(Methylenedi‐4,1‐phenylene) bismaleimide (BMI) is reported. BMI with two maleimides can simultaneously bind with FA⁺ and/or undercoordinated Pb²⁺ through chemical bonding, which also compresses the resultant perovskite lattice. The chemical bonding and strain modulation synergistically not only passivate film defects, but also inhibit perovskite decomposition, thus significantly improving the intrinsic stability of perovskite films. As a result, the BMI‐modified perovskite solar cells (PSCs) show improved power conversion efficiency (PCE) from 21.4% to 22.7% and enhanced long‐term operational stability, maintaining 91.8% of the initial efficiency after 1000 h under continuous maximum power point tracking. The findings shed light on the synergetic effects of chemical interactions and physical regulations, which opens a new avenue for stable and efficient perovskite‐based optoelectronic devices.