The present disclosure relates to methods and systems for calibrating an X-ray apparatus. The X-ray apparatus may include an X-ray detector and a collimator. To calibrate the X-ray apparatus, the methods and systems may include moving the X-ray detector from a first position to a second position along a first axis of a coordinate system, wherein the first position is under a scanning table, and the second position is outside the scanning table; moving the collimator to align the collimator with the X-ray detector at the second position; determining one or more parameters; and determining a second value of the first encoder when the collimator is aligned with the X-ray detector at the first position based on the one or more parameters.