For low-temperature solution-processed perovskite solar cells, ZnO as an electron transport layer (ETL) has been studied instead of TiO₂ requiring high sintering temperature. However reports have been scarce about successful fabrication of perovskite solar cells based on the solution-processed ZnO ETL under atmospheric air. In this study, we found a possible mechanism to cause poor performance of the perovskite solar cells, which can be ascribed to chemical reactions between the methylammonium iodide (MAI) of the perovskite precursor (MAPbI₃) solution and the ZnO film under humid conditions. The MAI solution could etch out ZnO ETL and promote serious carrier recombination if processed in air, whereas there was no such reaction for processing in a glovebox (H₂O < 0.1 ppm). In order to suppress the reactions in air-ambient, we applied a simple and novel dynamic spin-coating process, dripping the perovskite precursor solution on ZnO during the spin-coating process, followed by an anti-solvent washing treatment. Using this approach, for the first time, the interfacial reaction of ZnO/MAPbI₃ was significantly suppressed and thus a power conversion efficiency of the perovskite solar cells fully-processed in air-ambient was enhanced from 0% to 11.2%. These experimental results pave the way for the development of perovskite solar cells under low-temperature processing in air-ambient.