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Pure FAPbI3‐based, with FA being formamidinium, perovskite solar cells (PSCs) have garnered worldwide recognition for their exceptional efficiency. However, the phase stability of FAPbI3 is still a big obstacle in this area, because the ordinary strategy using MA+, Br−, Cs+ to stabilize α‐FAPbI3 phase can cause the bandgap change and ion migration. Herein, a new strategy is introduced to improve the α‐FAPbI3 phase stability by using a self‐assembled bridging layer at the buried interface of FAPbI3 perovskite in the n–i–p solar cell structure. A series of multidentate bisphosphonic acid molecules are screened and demonstrate that etidronic acid (EA) with the smallest steric hindrance behaves the best. The four P‐OH groups can first form multidentate anchors on SnO2 while the remaining unanchored ─OH and P═O groups can form strong interaction between I− and Pb2+. Thus, a strong and stable bridging layer is formed, which greatly increases the energy barrier of phase transition of FAPbI3. As a result, the pure FAPbI3‐based (MA+, Br+, Cs+‐free system) n–i–p device reached an impressive power conversion efficiency of 24.2% with good stability. Furthermore, the strong interaction between EA and Pb2+ can greatly reduce lead leakage in harsh conditions. [ABSTRACT FROM AUTHOR]