The present disclosure provides methods and systems for X-ray imaging. The methods may include obtaining pre-scan imaging data relating to a target section of a target subject. The methods may include determining, based on the pre-scan imaging data, a chord length of at least one chord of the target section. The methods may also include determining, based on the chord length of the at least one chord of the target section, exposure parameters to be used by the X-ray imaging device in a target scan of the target subject. The methods may further include reconstructing a target image of the target subject based on scan data collected by the X-ray imaging device in the target scan.