Amorphous selenium (a-Se) film is a promising photoconductive material for X-ray flat-panel detectors (FPD) application. However, a-Se tends to be crystalline Se at near room temperature. This remarkable temperature sensitivity limits its practical application. To prevent the near-room-temperature crystallization of a-Se film, we fabricate antimony (Sb)-doped a-Se films using a vacuum evaporation technique equipped with an in situ cooling trap. We experimentally demonstrate that the Sb doping improves the thermal stability of a-Se film while possessing a similar X-ray photoelectric conversion efficiency as the pure a-Se film. After air annealing at 50 °C for 90 min, the X-ray diffraction (XRD) results of the 4.1 at.% Sb-doped a-Se film shows no detectable crystallization diffraction peak. Upon applying an electric field of 10 V µm−1, such Sb-doped a-Se film exhibits dark current density below 1 nA cm−2, while under an X-ray dosage of about 4 mGy, the annealed Sb-doped a-Se film shows a photocurrent density of more than 100 nA cm−2.