The present disclosure provides a flat panel detector and a driving method thereof. A detection unit includes: a first transistor, a second transistor, a storage capacitor and a photoelectric detection device, and because an active layer of the second transistor is made of amorphous silicon semiconductor materials and an active layer of the first transistor is made of low-temperature poly-silicon semiconductor materials or metallic oxide semiconductor materials, transmission delay of an electric signal generated by the photoelectric detection device may be reduced by controlling conduction and cut-off of the first transistor and controlling conduction and cut-off of the second transistor.