Perovskite solar cells are promising highly efficient and low-cost photovoltaic devices. Nevertheless, the use of the toxic element Pb is still a big challenge for the mass production of highly efficient perovskite solar cells. In this work, a novel and facile technique is presented to fabricate tin-lead alloyed perovskite films using metal precursors and a substitution method. Herein, the perovskite films are formed as a result of the substitution reaction between the Sn metal precursor and methylammonium triiodideplumbate (CH₃NH₃PbI₃, MAPbI₃) solution carried out at 70 °C. The maximum Sn content in perovskite films reaches up to 15% with a substitution reaction time period of 1 day. Through this substitution technique and the unique growth mechanism, a less-toxic and efficient perovskite solar cell with a power conversion efficiency (PCE) of 12.46% is fabricated. It is found that this substituting strategy can exploit a new direction to further explore different active metals substituting less active organic metallic systems with even lower temperature conditions.