Organic-inorganic perovskites were treated with a CsI solution to improve the photovoltaic performance and stability. Due to the formation of CsPbI3 and trap filling in Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 perovskites by CsI treatment, the power conversion efficiency was improved to 20.48%. The CsI-treated perovskites exhibited slower degradation under heating at 180 °C than the untreated perovskites.