Abstract: To address the issues of carrier non-radiative recombination and device instability caused by surface defects in perovskite light-absorbing films of perovskite solar cells (PSCs), a multifunctional synergistic passivation strategy based on hexamethylammonium bromide (HMABr) was proposed. Theoretical calculations and experiments demonstrate that Br ⁻ in HMABr effectively passivates deep-level defect states induced by uncoordinated Pb ²⁺, while the quaternary ammonium cation HMA ⁺ anchors free I ⁻, suppressing ion migration. Additionally, the alkyl chain structure blocks moisture intrusion. After HMABr passivation, Defect density in perovskite films decreases significantly. Carrier lifetime increases from 620.94 ns to 706.44 ns. Device power conversion efficiency (PCE) improves from 22.21% to 23.39%. Unencapsulated devices retain 90.1% of initial efficiency after 1,152 hours in ambient air.