The binary tin and lead (Sn-Pb) perovskites are a significant cornerstone for all-perovskite tandem devices due to their high efficiency, low toxicity, and narrow bandgap properties. However, the fast crystallization of the perovskite film and the facile conversion of Sn²⁺ to Sn⁴⁺ during the preparation process are major obstacles to achieving remarkable Sn-Pb perovskite solar cells (PSCs). Herein, we develop a multifunctional anion-cation modulation strategy to synthesize high-quality FAPb_0.5Sn_0.5I₃ (E_g~1.24 eV) perovskite films by mixing methylamine acetate (MAAc) ionic liquid with the PbI₂ and SnI₂ solutions through a two-step sequential deposition process. MA cations tend to insert the tin/lead-halogen octahedral framework and occupy the A-site in the first-step. Subsequently, the volatile MA cations can produce ion exchange reactions with FA⁺ from the formamidinium iodide in the second step, leading to high quality crystallization process. Additionally, the Ac anions can efficiently suppress Sn²⁺ oxidation with balancing the crystallization rate of the Pb/Sn components, and passivate the defect sites located on the grain boundary and surface, owing to the strong coordination of Ac^- with Sn²⁺ and Pb²⁺. As a result, the optimized PSC reaches a power-conversion efficiency of 21.22 %, which is the highest value reported for FAPb0.5Sn0.5I3 based solar cells with a two-step method, compared to the control device with 12.86 %. Moreover, the perovskite devices show prominent reproducibility and stability performance, maintaining >80 % of the initial efficiency after being stored in the N₂-filled glovebox for 1000 h.