A method for modulating x-ray radiation dose in CT scan is disclosed. The method may include: acquiring x-ray attenuation information of each Z position within a planned scanning sequence for a scan region of a subject from data of a finished scanning sequence for the subject and then, based on the acquired x-ray attenuation information, performing dose modulation with respect to the XY scanning profile at each Z position within the planned scanning sequence. The finished scanning sequence can be an axial and/or helical scanning sequence for a scan region wholly or partially same as the scan region and has been performed earlier. The Z position may include a position in Z direction extending from head to foot of the subject or conversely. The XY scanning profile may include a scanning profile of the subject which is vertical to Z direction.
