In this paper, high‐photosensitive CdS1–xSex films are synthesized by a two‐step technique, which includes the chemical bath deposition of CdS films and a following selenization process. The structural, optical, and photoelectric properties of the CdS1–xSex films were investigated. With the substitution of selenium for sulfur atoms, grain sizes of the as‐prepared CdS1–xSex films are effectively enlarged and reach the scales of the films thickness when the selenization temperature exceeds 450 °C. With increasing the selenization temperature from 350 to 550 °C, the band gaps of CdS1–xSex films gradually decrease from 2.37 to 1.82 eV. Under the co‐action of the grain‐size enlargement and band‐gap decrease, the CdS1–xSex films fabricated at 450 °C show very pronounced photosensitivity. Noteworthy, the ratio of photo to dark conductivity of the CdS1–xSex film selenized at 450 °C reaches 1.1 × 105, suggesting a promising application potential in the photoelectric devices.