Tin-based halide perovskite materials have been successfully employed in lead-free perovskite solar cells, but the overall power conversion efficiencies (PCEs) have been limited by the high carrier concentration from the facile oxidation of Sn to Sn . Now a chemical route is developed for fabrication of high-quality methylammonium tin iodide perovskite (MASnI ) films: hydrazinium tin iodide (HASnI ) perovskite film is first solution-deposited using presursors hydrazinium iodide (HAI) and tin iodide (SnI ), and then transformed into MASnI via a cation displacement approach. With the two-step process, a dense and uniform MASnI film is obtained with large grain sizes and high crystallization. Detrimental oxidation is suppressed by the hydrazine released from the film during the transformation. With the MASnI as light harvester, mesoporous perovskite solar cells were prepared, and a maximum power conversion efficiency (PCE) of 7.13 % is delivered with good reproducibility.