High-quality formamidinium lead iodide (FAPbI3) perovskite thin films are fabricated via organic cation exchange. With ammonia lead iodide (NH4PbI3) as the starting material, the NH4(+) in NH4PbI3 could be gradually substituted by FA(+) in formamidine acetate (FA-Ac) and simultaneously transformed to the pure phase α-FAPbI3 at elevated temperature.