Formamidinium lead triiodide (α-FAPbI) has been widely used in high-efficiency perovskite solar cells due to its small band gap and excellent charge-transport properties. Recently, some additives show facet selectivity to generate a (001) facet-dominant film during crystallization. However, the mechanism to realize such (001) facet selectivity is not fully understood. Here, the authors attempted to use three ammonia salts NHX (X are pseudohalide anions) to achieve better (001) facet selectivity in perovskite crystallization and improved crystallinity. After addition, the (001) facet dominance is generally increased with the best effect from SCN anions. The theoretical calculation revealed three mechanisms of such improvements. First, pseudohalide anions have larger binding energy than the iodine ion to bind the facets including (110), (210), and (111), slowing down the growth of these facets. The large binding energy also reduces nucleation density and improves crystallinity. Second, pseudohalide ions improve phase purity by increasing the formation energies of the δ-phase and other hexagonal polytypes, retarding the α- to δ-phase transition. Third, the strong binding of these anions can also effectively passivate the iodine vacancies and suppress nonradiative recombination. As a result, the devices show a power conversion efficiency of 24.11% with a V of 1.181 V.