The present invention relates to a flat panel X-ray detector, which comprises a thin film transistor (TFT) substrate; a photoelectric detecting layer, which is disposed on and electrically connected with the TFT substrate, wherein the photoelectric detecting layer comprises a plurality of photoelectric detecting units and a plurality of light absorption units, and the light absorption unit is disposed between spaces adjacent to the photoelectric detecting unit; a Scintillation layer, which is disposed on the photoelectric detecting layer; and a reflective layer, which is disposed on the Scintillation layer.