AbstractCompared to widely adopted low‐dimensional/three‐dimensional (LD/3D) heterostructure, functional organic cation based surface termination on perovskite can not only realize advantage of defect passivation but also prevent potential disadvantage of the heterostructure induced intercalation into 3D perovskite. However, it is still very challenging to controllably construct surface termination on organic–inorganic hybrid perovskite because the functional organic cations’ substitution reaction is easy to form LD/3D heterostructure. Here, we report using a novel benzyltrimethylammonium (BTA) functional cation with rational designed steric hindrance to effectively surface terminate onto methylammonium lead triiodide (MAPbI₃) perovskite, which is composed of the most unstable MA cations. The BTA cation is difficult to form a specific 1.5‐dimensional perovskite of BTA₄Pb₃I₁₀ by cation substitution with MA cation, which then provides a wide processing window (around 10 minutes) for surface terminating on MAPbI₃ films. Moreover, the BTAI surface terminated BTAI‐MAPbI₃ shows better passivation effect than BTA₄Pb₃I₁₀‐MAPbI₃ heterojunction. Finally, BTAI surface terminated solar cell (0.085 cm²) and mini‐module (11.52 cm²) obtained the efficiencies of 22.03 % and 18.57 %, which are among the highest efficiencies for MAPbI₃ based ones.