Formamidinium lead iodide (FAPbI3) is a category of perovskite material with an ideal band gap and high thermal stability, which can be efficiently prepared by two-step spin-coating. Spin-coating organic salts and transforming intermediate phase at the second step involves a components' reaction and state transition, thus playing a crucial role in the film quality formed afterward and optoelectronic properties of the fabricated perovskite solar cells (PSCs). In this paper, a cooling stage (CO) is used to post-treat the as-prepared precursor after the second spin-coating. The procedure of intermediate phase transferring to other state is found to be retarded; hence, the appearing velocity of perovskite nucleation is decreased. As a result, components react more adequately and larger perovskite grains with fewer defects are obtained; charge transport as well as carrier recommbination behaviors are therefore optimized. The PSCs based on the CO process achieved a champion power conversion efficiency (PCE) of 21.51% with enhanced stability. Moreover, CO treatment is observed to be beneficial for improving the film quality of perovskite in large-area preparation, which we anticipate can be further extended to the commercialized application of PSCs.