Perovskite films prepared with CH3NH2 molecules under ambient conditions have led to rapid fabrication of perovskite solar cells (PSCs), but there remains a lack of mechanistic studies and inconsistencies with operability in their production. Here the crystal structure of CH3NH2-CH3NH3PbI3 was analyzed to involve hydrogen bonds (CH3NH2 center dot center dot center dot CH3NH3+) and has guided the facile, reproducible preparation of high-quality perovskite films under ambient conditions. Hydrogen bonds within CH3NH2 center dot center dot center dot CH3NH3+ dimers were found in the CH3NH2-CH3NH3PbI3 intermediates, accompanied by 1D-PbI3- chains (delta-phase). The weakly hydrogen-bonded CH3NH2 molecules were easily released from the CH3NH2-CH3NH3PbI3 intermediates, contributing to rapid, spontaneous phase transition from 1D-PbI3- (delta-phase) to 3D-PbI3- (alpha-phase). Further introduction of CH3NH3Cl into the CH3NH2-CH3NH3PbI3 intermediates led to interruption of 1D-PbI3- transition into 0D-Pb2I9-xClx5-(0 < x < 6), adjusting the phase transition route toward 3D-PbI3-. On the basis of the above understanding, CH3NH2 solution in ethanol and CH3NH3Cl were used for precursors and a best efficiency of 20.3% in PSCs was achieved. Large-scale modules (12 cm(2) aperture area) fabricated by a dip-coating technology exhibited an efficiency up to 16.0% and outstanding stability over 10 000 s under continuous output. The developed preparation method of perovskite precursors and insightful research into the methylamine-dimer-induced phase transition mechanism have enabled the production of high-quality perovskite films with robust operability, showing great potential for large-scale commercialization.