Environment-friendly Tin (Sn)-based perovskite solar cells (PSCs) have lately made significant development, showing tremendous promise in addressing the hazardous problems associated with Pb-based PSCs. However, even in N2 atmospheres, the thermodynamic stability of Sn-based perovskite films and long-term stability of Sn-based PSCs are demonstrated to be poor due to the presence of interfacial defect trap states. Here, we demonstrate the post-treatment of Sn-based perovskite films with ethylenediamine formate (EDAFa2) ion salt, serving as a bi-functional interface layer to in situ passivate the interfacial defect and improve the stability of Sn2+ by creating a thermodynamic chemical environment pathway. Moreover, the presence of EDAFa2 is shown to promote the interfacial energy level alignment, which is beneficial for the charge extraction at the interface. As a result, PSC devices with a bi-functional interface achieve a champion power conversion efficiency (PCE) as high as 9.40% and enhanced stability, retaining ∼95% of the original PCE stored in a N2 environment after ∼1960 h without encapsulation. This work highlights the significant role of an interfacial design in efficient and stable Sn-based PSCs.