The emerging inorganic CsPbI(3)perovskites are promising wide-bandgap materials for application in tandem solar cells, but they tend to transit from a black alpha phase to a yellow delta phase in ambient conditions. Herein, a gradient grain-sized (GGS) CsPbI(3)bilayer is developed to stabilize the alpha phase via a single-step film deposition process. The spontaneously upward migration of (adamantan-1-yl)methanammonium (ADMA) based on the hot-casting technique causes self-assembly of the hierarchical morphology for the perovskite layers. Due to the strong steric effect of the surficial ADMA cation, a self-assembly tiny grain-sized CsPbI(3)layer is in situ formed at the surface site, which exhibits notably enhanced phase stability by its high surface energy. Meanwhile, a large grain-sized CsPbI(3)layer is obtained at the bottom site with high charge mobility and low trap density of states, which benefits from the regulated growth rates by the interaction between ADMA and perovskites. The perovskite solar cell (PSC) based on the GGS CsPbI(3)bilayer shows an efficiency of 15.5% and operates stably for 1000 h under ambient conditions. This work confirms that redistributing the surface energy of perovskite films is a facile strategy to stabilize metastable PSCs without the cost of efficiency loss.