Precise control over cation distribution is critical for high‐performance perovskite solar cells (PSCs). Conventional bulk doping often leads to vertical segregation and lattice strain, while surface passivation dose not ensure bulk homogeneity. We introduce a triple‐alkali interlayer (LiOH/KCl/CsI) deposited on the electron transport layer prior to crystallization of the perovskite film. This design spatially decouples crystallization regulation from compositional modulation, i.e., localized Li + and K + ions reconstruct the buried contact and passivate defects and interfacial Cs + acts as a dynamic source for in situ upward diffusion. This bottom‐up mechanism facilitates stress‐free crystallization, resulting in a dense, preferentially oriented perovskite film with a void‐free buried interface and superior compositional homogeneity. Consequently, the resulting champion n‐i‐p PSC achieves a remarkable power conversion efficiency of 26.13%, with a high open‐circuit voltage of 1.184 V and a fill factor of 83.81%. Furthermore, the devices demonstrate robust durability maintaining 93.7% after 1440 h of continuous 1‐sun irradiation at 65°C. This work provides a promising pathway for managing cation dynamics to realize efficient and stable perovskite photovoltaics.