To fabricate stable neat FAPbI(3) perovskite with a pure alpha-phase (pure alpha-FAPbI(3)) is important in the field of photovoltaic commercialization because of its better bandgap than its alloying counterpart with cesium (Cs) or methylammonium (MA) cations. In this work, the first vapor deposited pure alpha-FAPbI(3) thin film solar cell with a power conversion efficiency (PCE) over 20% is achieved by regulating the phase transition process. It is found that under high humidity conditions, a fast phase transition between high-purity alpha- and delta-phase FAPbI(3) can be realized. Moreover, theoretical calculations interestingly reveal a phase transition shortcut induced by the abnormal volume contraction that is ascribed to the formation of double hydrogen bonds at a certain H2O concentration. Therefore, a high-humidity post-treatment strategy is proposed to fabricate alpha-FAPbI(3) solar cells with a champion PCE of 20.19% (0.1 cm(2)) and 18.91% (1 cm(2)), which is currently the highest recorded value in vapor deposited pure alpha-FAPbI(3) perovskite solar cells. This study helps to redefine the effect of a water molecule on FAPbI(3) solar cells. In addition, the demonstrated scaling-up possibility provides another perspective for fabricating uniform high-performance pure alpha-FAPbI(3) perovskite solar cells.