Flexible perovskite solar cells have attracted much attention in the scientific community due to their lightweight nature, high flexibility, and superior power-to-mass ratio. One of the most effective strategies for enhancing the power conversion efficiency of these cells involves addressing grain boundary defects within the perovskite films and interfacial defects between the perovskite films and charge transport layers. In this work, we optimize the performance of inverted flexible perovskite solar cell by using octadecylamine hydrochloride (OACl) as both an additive and a surface passivating agent to achieve synergistic passivation to the bulk phase and surface. The incorporation of OACl in the perovskite precursor solution results in the enlarging of the perovskite crystal grains, enhancing crystallinity, and passivating of grain boundary defects within the perovskite film. This optimization leads the open-circuit voltage to increase from 1.07 to 1.12 V, fill factor from 70.86% to 75.04%, and power conversion efficiency from 18.08% to 20.12%. In addition, the OACl solution is used to passivate the surface of perovskite film, resulting in a smoother perovskite surface, fill the grain boundaries, and reduce the defect density on the perovskite surface. As a result, the optimized device exhibits an open-circuit voltage of 1.15 V, fill factor of 76.15%, and ultimately achieves a power conversion efficiency of 20.80% for flexible perovskite solar cells. The synergistic passivation strategy based on OACl used in this work provides an effective approach for fabricating efficient flexible perovskite solar cells.