AbstractTransition of δ‐phase formamidinium lead triiodide (δ‐FAPbI3) to pure α‐phase FAPbI3 (α‐FAPbI3) typically requires high processing temperature (150 °C), which often results in unavoidable residual stress. Besides, using methylammonium chloride (MACl) as additive in fabrication will cause MA residue in the film, compromising the compositional purity. Here, a stress‐released and compositional‐pure α‐FAPbI3 thin‐film is fabricated using 3‐chloropropylammonium chloride (Cl‐PACl) by two‐step annealing. The 2D template of n = 2 can preferentially form in perovskite with the introduction of Cl‐PACl at a temperature as low as 80 °C. Such a 2D template can guide the free components to form ordered α‐FAPbI3 and promote the transition of the formed δ‐FAPbI3 to α‐FAPbI3 by reducing the phase transition energy. As a result, the obtained perovskite films via low‐temperature phase‐transition have a high degree of crystal orientation and reduced residual stress. More importantly, most of the Cl‐PACl is volatilized during the subsequent high‐temperature annealing process accompanied by the disintegration of the 2D templates. The residual trace of Cl‐PA+ is mainly concentrated at the grain boundary near the perovskite surface layer, stabilizing α‐FAPbI3 and passivating defects. Perovskite solar cell based on pure α‐FAPbI3 achieves a power conversion efficiency of 23.03% with excellent phase stability and photo‐stability.