Spatially uniform high-temperature superconducting films are highly desirable for exploring novel properties and popularizing applications. To improve the uniformity, we fabricate monolayer FeSe_xTe_1−x (0 < x ≤ 1) films on SrTiO₃(001) by topotactic reaction of monolayer FeTe films with selenium. Using in situ low-temperature scanning tunneling microscopy/spectroscopy, we demonstrate atomic-level uniformity of element distribution and well-defined superconducting gaps of ~ 15 meV in FeSe_xTe_1−x films. In particular, the monolayer FeSe films exhibit fewer line defects and higher superfluid density as evidenced by sharper coherence peaks than those prepared by the co-evaporation method. Our results provide a promising way to optimize sample quality and lay a foundation for studying new physics and drawing reliable conclusions.