Two-step-fabricated FAPbI3-based perovskites have attracted increasing attention because of their excellent film quality and reproducibility. However, the underlying film formation mechanism remains mysterious. Here, the crystallization kinetics of a benchmark FAPbI3-based perovskite film with sequential A-site doping of Cs+ and GA+ is revealed by in situ X-ray scattering and first-principles calculations. Incorporating Cs+ in the first step induces an alternative pathway from δ-CsPbI3 to perovskite α-phase, which is energetically more favorable than the conventional pathways from PbI2. However, pinholes are formed due to the nonuniform nucleation with sparse δ-CsPbI3 crystals. Fortunately, incorporating GA+ in the second step can not only promote the phase transition from δ-CsPbI3 to the perovskite α-phase, but also eliminate pinholes via Ostwald ripening and enhanced grain boundary migration, thus boosting efficiencies of perovskite solar cells over 23%. This work demonstrates the unprecedented advantage of the two-step process over the one-step process, allowing a precise control of the perovskite crystallization kinetics by decoupling the crystal nucleation and growth process.