Tin-based perovskite solar cells (PSCs) are potential light absorbers for solar cell applications since they are less toxic compared to commonly used lead-based alternatives. Retaining the less stable Sn2+ state is key to improving the efficiency of tin-based PSCs. Organic Lewis base molecules have demonstrated potential to achieve this purpose. However, the critical factors influencing the performance of Lewis bases are largely unknown. In this study, we applied density functional theory (DFT) to investigate seven Lewis base materials, including methanol (MeOH), dimethyl ether (DME), ethyl methyl ether (EME), methyl acetate (MeOAc), methyl ammonium (MA), methyl sulfonic acid (MSA), and methyl phosphonic acid (MPA). Our results show that the effectiveness of passivation is linked to the gap between the HOMO and the LUMO (Egap). These findings provide theoretical guidance to screen Lewis base additives for enhancing energy conversion efficiencies of tin-based PSCs.