The exceptional photovoltaic properties of Sn-based perovskite solar cells (Sn–PSCs) have caught the interest of researchers as a potential candidate for the development of non-toxic PSCs. Nevertheless, the Sn–PSCs still face some critical drawbacks, such as the easier oxidation of Sn²⁺ to Sn⁴⁺ and the moisture permeation, which reduce their stability and power conversion efficiency (PCE). In this work, to tackle these limitations, we added a Lewis base additive formamidinesulfinic acid (FASO₂H) to the perovskite precursor solution. The FASO₂H Lewis base additive could successfully reduce the oxidation of Sn²⁺/Sn⁴⁺ by 32%, improved the crystallinity and decreased the lattice strain by 66% in the respective films. As a result, the charge carrier recombination minimized and the FASnI₃-FASO₂H (3 mol%) based PSCs produced a PCE of 7.44%. The FASO₂H additive also offers superior moisture protection for Sn–PSCs thanks to its hydrophobic sulfinic acid group. Notably, the Sn–PSCs exhibit a high light soaking stability by maintaining 90% of the initial PCE up to 450 h when tested under maximum power point tracking conditions.