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Perovskite solar cells have received worldwide interests due to swiftly improved efficiency but the poor stability of the perovskite component hampers the device fabrication under normal condition. Herein, we develop a reliable nonstoichiometric acid-base reaction route to stable perovskite films by intermediate chemistry and technology. Perovskite thin-film prepared by nonstoichiometric acid-base reaction route is stable for two months with negligible PbI 2 -impurity under ∼65% humidity, whereas other perovskites prepared by traditional methods degrade distinctly after 2 weeks. Route optimization involves the reaction of PbI 2 with excess HI to generate HPbI 3 , which subsequently undergoes reaction with excess CH 3 NH 2 to deliver CH 3 NH 3 PbI 3 thin films. High quality of intermediate HPbI 3 and CH 3 NH 2 abundance are two important factors to stable CH 3 NH 3 PbI 3 perovskite. Excess volatile acid/base not only affords full conversion in nonstoichiometric acid-base reaction route but also permits its facile removal for stoichiometric purification, resulting in average efficiency of 16.1% in forward/reverse scans.