A light detection device includes a first gate line, a second gate line adjacent to the first gate line, a first switching unit, a second switching unit, and a readout circuit. The first switching unit includes a first gate electrode connected to the first gate line, and a first drain electrode electrically connected to a light sensing unit. The second switching unit includes a second gate electrode connected to the second gate line, and a second drain electrode electrically connected to a light sensing unit. An operation method of the light detection device includes receiving a first readout signal and a second readout signal from the first switching unit and the second switching unit respectively by the readout circuit and calculating a difference between the first readout signal and the second readout signal for determining whether the light detection device is irradiated by detection light.