Driven by their many unique features, perovskite solar cells (PSCs) have become one of the most promising candidates in the photovoltaic field. Two-step preparation of perovskite film is advantageous for its higher stability and reproducibility compared to the one-step method, which is more suitable for practical application. However, the incomplete conversion of the dense lead iodide (PbI₂) layer during the sequential spin-coating of formamidinium/methylammonium (FA⁺/MA⁺) organic amine salts severely affect the performance of PSCs. Herein, sodium bicarbonate (NaHCO₃) is used to induce the formation of porous PbI₂, which facilitates the penetration of the FA⁺/MA⁺ ions and the formation of a perovskite film with high crystallinity and large grain microstructure. Meanwhile, the introduction of Na⁺ not only improves the energetic alignment of the PSC, but also increases the conductivity via p-doping. As a result, the optimized NaHCO₃-modified PSC achieves a champion power conversion efficiency of 24.0% with suppressed hysteresis. Moreover, the significant reduction in defect density and ion migration as well as a mild alkaline environment enhance the stability of device. The unencapsulated NaHCO₃-modified PSCs maintain over 90% of their original efficiency upon storage in ambient air (30%–40% relative humidity) for 2160 h. We have demonstrated an ingenious strategy for controlling the quality of perovskite and improving the performance of device by low-temperature foaming of simple inorganic molecules of NaHCO₃. (Figure presented.).