Abstract As a next‐generation photovoltaic device, perovskite solar cells are rapidly emerging. Nevertheless, both solution and device stability pose challenges for commercialization due to chemical degradation caused by internal and external factors. Especially, the decomposition of iodoplumbate in a perovskite solution hinders the long‐term use of perovskite solutions. Moreover, the synthesis of stable perovskites at low temperature is important for stable devices and wide applications (flexible devices and high reproducibility). Herein, the critical composition of perovskite is found to obtain high stabilities of both iodoplumbate and perovskite crystals by utilizing CsPbBr 3 and FAPbI 3 , exhibiting high device performance and long‐term solution storage. The novel composition of CsPbBr 3 ‐alloyed FAPbI 3 not only crystallizes under annealing‐free conditions but also demonstrates excellent iodoplumbate stability for 100 days (∼3000 h) without any degradation. Furthermore, high device stabilities are achieved over 2000 and 3000 h under extreme conditions of A.M. 1.5 and 85°C/85% relative humidity, respectively. Overall, the device exhibited a high power conversion efficiency of 23.4%, and furthermore, CsPbBr 3 ‐alloyed FAPbI 3 was devoted to widen the applications in both flexible and carbon‐electrode devices, thereby addressing both scientific depths and potential commercial materials.