Triple‐cation mixed perovskites have attracted much attention recently owing to their prominent optoelectronic properties and good stability for perovskite solar cells. However, the introduction of those cations with different sizes in the perovskite materials will drive the perovskite lattice away from ideal cubic structure and lead to microstrain in the resultant films. Herein,a small amount of KBF4 as an additive to elevate the quality of triple‐cation mixed perovskite thin films is introduced. It is found that KBF4 can enhance the crystallinity and alleviate microstrain of the perovskite thin films. Moreover, KBF4 can passivate defects in perovskite grains, leading to much longer carrier lifetimes. Consequently, the resultant devices show improved fill factor, enhanced device efficiency, and better device stability. Under optimum fabrication conditions, triple‐cation mixed perovskite solar cells with an inverted structure show power conversion efficiency over 23% as well as excellent stability under different conditions. High‐performance inverted perovskite solar cells are fabricated with KBF4 additive in perovskite layers. The introduction of KBF4 can enlarge grain size, manipulate microstrain, and suppress the formation of deep trap states in the perovskite layers. The synergistic effects greatly elongate carrier lifetime and enable power conversion efficiencies over 23% and 21% for rigid and flexible devices.