A pixel circuit of an X-ray sensor includes a photo diode, a first transistor, a second transistor and a third transistor. The photo diode is used to sense X-rays and to generate a corresponding electrical sensing signal. The first transistor is electrically connected with the photo diode to reset the electrical sensing signal. The second transistor is electrically connected with the photo diode to amplify the electrical sensing signal and to generate an amplified sensing signal. The third transistor is electrically connected with the second transistor to output the amplified sensing signal. The second transistor has a terminal electrically connected with a high voltage, and the first transistor has a terminal electrically connected with a calibration voltage. The high voltage and the calibration voltage are controlled separately.