Electrochemical reduction of carbon dioxide (CO 2 ER) has become an effective solution to relieve the energy crisis and tackle climate change. In this study, a series of tin-based organic frameworks modified by In (Sn-MOF/In x ) were successfully synthesized via a simple hydrothermal method and explored for high formate-selective CO 2 ER. The pure Sn-MOF exhibits maximum formate selectivity with a faradaic efficiency (FE formate ) of approximately 85.0% and a current density of 15 mA cm -2 at -1.16 V RHE , while the In (6%)-modified Sn-MOF (Sn-MOF/In 6 ) delivers a much higher maximum FE formate (around 97.5%) and a current density of 16 mA cm -2 at -0.96 V RHE . Remarkably, the Sn-MOF/In 6 exhibits a significantly larger specific surface area (183.3 m 2 g -1 ) compared to the Sn-MOF (65.2 m 2 g -1 ). These findings indicate that introducing In, an alien element with a slightly different outer orbital electron number from that of Sn, can significantly boost the selectivity and activity for CO 2 ER to formate. This study presents an efficient way to modify MOF catalysts through a well-designed introducing process.