AbstractWide‐bandgap (WBG) perovskite solar cells (PSCs) have been widely used as the top cell of tandem solar cells. However, photoinduced phase segregation and high open‐circuit voltage loss pose significant obstacles to the development of WBG PSCs. Here, a two‐step small‐size A‐site and large‐size X‐site incorporation strategy is reported to modulate the lattice distortion and improve the film quality of WBG formamidinium‐methylammonium (FAMA) perovskite films for photostable PSCs based on two‐step deposition method. First, CsI with content of 0–20% is introduced to tune the lattice distortion and film quality of FAMA perovskite with a bandgap of 1.70 eV. Then, 4% RbI is incorporated to further modulate the perovskite growth and lattice distortion, leading to the suppression of photoinduced phase segregation in the resultant RbCsFAMA quadruple cation perovskites. As a result, the 20%CsI/4%RbI‐doped device obtains a promising efficiency of 20.6%, and the corresponding perovskite film shows good photothermal stability. Even without encapsulation, the device can maintain 92% of its initial efficiency after 1000 h of continuous operation under 1 sun equivalent white light‐emitting diode illumination.