Formamidinium lead triiodide (FAPbI3) perovskite has received great attention because of its distinct optoelectronic property, smaller band gap (~1.5 eV), and higher thermal stability than methylammonium lead triiodide (MAPbI3). However, the efficient synthesis of such perovskite materials on a large scale at a low cost remains a challenge. In this work, mixed-cation FA1−xCsxPbI3 thin films were directly prepared in an atmospheric environment with a high film formation rate, low material loss, low cost, and low requirements for experimental instruments and environment. The as-obtained FAPbI3 films exhibited excellent optoelectronic properties, showing promising applications in the photodetection field.