A flat panel detector includes a base substrate, a sensing electrode and a bias electrode over the base substrate, and an insulating layer over the sensing electrode and the bias electrode at a side distal from the substrate. A difference between thicknesses of regions of the insulating layer corresponding to the sensing electrode and the bias electrode respectively is not greater than a preset threshold. When a sufficiently high voltage is applied to the insulating layer and turned on, because the thickness thereof is relatively uniform, a dark current generated by the sensing electrode and the bias electrode under the insulating layer is relatively uniform, thereby improving detection accuracy of the flat panel detector.