High power conversion efficiency (PCE) flexible perovskite solar cells (FPSCs) are highly desired power sources for aerospace crafts and flexible electronics. However, their PCEs still lag far behind their rigid counterparts. Herein, we report a high PCE FPSC by controllable growth of a SnO₂ electron transport layer through constant pH chemical bath deposition (CBD). The application of SnSO₄ as tin source enables us to perform CBD without strong acid, which in turn makes it applicable to acid-sensitive flexible indium tin oxide. Furthermore, a mild and controllable growth environment leads to uniform particle growth and dense SnO₂ deposition with full coverage and reproducibility, resulting in a record PCE of up to 25.09% (certified 24.90%) for FPSCs to date. The as-fabricated FPSCs exhibited high durability, maintaining over 90% of their initial PCE after 10000 bending cycles.