Sn-based perovskite solar cells have been considered to be promising alternatives for toxic Pb-based perovskite solar cells due to their comparable optoelectronic properties as well as relatively lower toxicity. However, their poor oxidation stability has been a main obstacle for efficient Sn-based perovskite solar cells. Herein, we fabricated quasi-2D Sn-based perovskite solar cells with improved stability and studied the effects of formamidinium thiocyanate (FASCN) additive which can interact with Sn²⁺ ion to suppress oxidation. The carrier density of the device with FASCN was reduced to 6.67×1015 cm^-3 from 2.17×1016 cm^-3. Moreover, morphological improvement was observed. The best-performing sn-based perovskite solar cells showed the highest effciency of 8.17%, which retains over 90% of its initial efficiency over 1000 hours in a nitrogen-filled glove box. These results demonstrate a viable approach for efficient Sn-based perovskite solar cells.