Phase instability is one of the major obstacles to the wide application of formamidinium (FA)‐dominated perovskite solar cells (PSCs). An in‐depth investigation on relevant phase transitions is urgently needed to explore more effective phase‐stabilization strategies. Herein, the reversible phase‐transition process of FA1−xCsxPbI3 perovskite between photoactive phase (α phase) and non‐photoactive phase (δ phase) under humidity, as well as the reversible healing of degraded devices, is monitored. Moreover, through in situ atomic force microscopy, the kinetic transition between α and δ phase is revealed to be the “nucleation–growth transition” process. Density functional theory calculation implies an enthalpy‐driven α‐to‐δ degradation process during humidity aging and an entropy‐driven δ‐to‐α healing process at high temperatures. The α phase of FA1−xCsxPbI3 can be stabilized at elevated temperature under high humidity due to the increased nucleation barrier, and the resulting non‐encapsulated PSCs retain >90% of their initial efficiency after >1000 h at 60 °C and 60% relative humidity. This finding provides a deepened understanding on the phase‐transition process of FA1−xCsxPbI3 from both thermodynamics and kinetics points of view, which also presents an effective means to stabilize the α phase of FA‐dominated perovskites and devices for practical applications.