State-of-the-art high-performance perovskite solar cells are mainly based on formamidinium (FA)-dominated perovskites because of their narrow band gap and remarkable thermal resistance. However, photoactive α-FAPbI is prone to transit to the photoinactive phase, and pioneering phase stabilization strategies can induce undesirable band gap broadening or phase segregation, seriously restricting the efficiency and long-term stability of the resultant photovoltaics. Herein, a small molecule of ammonium acetate (NHAc) was introduced as an additive in a modified ripening method to fabricate component-pure α-FAPbI. Owing to the strong interaction between NHAc and PbI, FAI via Pb-O coordination, and N-H···N hydrogen bonding, vertically oriented perovskites with relaxed crystal strain were first generated, which were fully converted to α-FAPbI in a further ripening process. The NHAc was fully volatized after the perovskite formation, resulting in component-pure α-FAPbI with a band gap of 1.48 eV and remarkable stability under light illumination. Ultimately, a champion device efficiency of above 21% was obtained based on the component-pure α-FAPbI and over 95% of the initial efficiency can be maintained after 1000 h of aging.