A flat panel detector substrate includes a base substrate, a photoelectric conversion layer, a bias signal line, and a conductive structure. The photoelectric conversion layer is located on the base substrate. The photoelectric conversion layer has a first surface proximate to the base substrate and a second surface away from the base substrate. The bias signal line is located between the photoelectric conversion layer and the base substrate. The conductive structure is located on the base substrate. One end of the conductive structure is coupled to the second surface of the photoelectric conversion layer, another end of the conductive structure is coupled to the bias signal line, and a portion therebetween is located beside the photoelectric conversion layer.