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The Achilles heel of the perovskite solar cells (PSCs) is the long‐term stability under working condition which restricts the commercialization. There are many causes for the poor stability including intrinsic defects in perovskite and humidity‐induced degradation. This work systematically investigates the synergistic working mechanism of defect passivation and humidity erosion protection on organic‐inorganic metal‐halide perovskite with benzyldodecyldimethylammonium bromide (BDDAB). The functionalized parts of halogen anions (Br−) can simultaneously passivate defects and improve the resistance of the humidity erosion by repairing [PbI6]4− octahedron defects as well as providing hydrophobicity. Moreover, the benzene ring in BDDAB tends to form π–π stacking with the benzene ring in the hole transport layer, which increases the intermolecular interaction. Thus, the highest efficiency of 22.08% is obtained by BDDAB‐modified PSCs, and 93.29% initial efficiency is retained after aging for 500 h at 80% relative humidity. [ABSTRACT FROM AUTHOR]