Organic ammonium salts have been widely used for defect passivation to suppress nonradiative charge recombination in perovskite solar cells (PSCs). However, they are prone to form undesirable in‐plane favored 2D perovskites with poor charge transport capability that hamper device performance. Herein, the defects passivation role of alkyldiammonium including 1.6‐hexamethylenediamine dihydriodide (HDAI2), 1,3‐propanediamine dihydriodide (PDAI2), and 1.4‐butanediamine dihydriodide (BDAI2) for formamidinium‐cesium perovskite is systematically investigated. With help of density functional theory (DFT) calculations, BDA with suitable size can synergistically passivate two defect sites on perovskite surfaces, showing the best defect passivation effect among the above three alkyldiammonium salts. Perovskite films based on BDAI2 modification are found to keep the 3D perovskite phase with considerably reduced trap‐state density, and enhanced carrier extraction. As a result, the BDAI2‐modified devices deliver impressive efficiencies of 23.1% and 20.9% for inverted PSCs on the rigid and flexible substrates, respectively. Moreover, the corresponding encapsulated rigid devices maintain 92% of the initial efficiency after operating under continuous 1‐sun illumination with the maximum power point tracking for 1000 h. Furthermore, the mechanical flexibility of the BDAI2‐modified flexible device is also improved due to the release of residual stress. BDA can make full use of the two ammonium cations for passivation and strengthen the absorption of BDA onto the VFA defect as well as enhance the formation energy of VFA, and thereby anchor the perovskite surfaces, so as to improve the photovoltaic performance of rigid and flexible devices.