Formamidinium‐cesium triiodide (FA_xCs_1‐xPbI₃) perovskite exhibits excellent phase stability, making it the most promising candidate for commercial perovskite solar cell (PSC) applications, particularly those with inverted structures present a promising contribution to the field of perovskite production. However, this composition often forms small grain sizes and has a large number of defects and PbI₂ residues on its surface, which can damage device performance. In this study, a post‐surface engineering strategy called the “clean‐passivation” method is proposed to address the interfacial problem between the perovskite and the electron transport layer (ETL). This method significantly reduces surface and grain boundary defects and eliminates unreacted PbI₂, resulting in suppressed iodine decomposition and ion migration during operation. As a result, an excellent power conversion efficiency of 24.27% with superior stability is achieved, as the unencapsulated device maintains 97.12% of its initial efficiency after 1500 h of continuous light soaking. Furthermore, this new surface clean‐passivate strategy can be universally applied to other typical perovskite compositions.