1. A method for controlling an X-ray dose of a CT scan, the method comprises: setting an initial X-ray dose; performing a first CT scan with the initial X-ray dose to obtain an initial scan image; setting a region of interest (ROI) in the initial scan image; calculating a subsequent X-Ray dose with image values of the ROI in the initial scan image; performing an additional CT scan with the calculated subsequent X-Ray dose to obtain an average image; repeating the following operations before receiving an end instruction: recalculating a subsequent X-Ray dose with image values of the ROI in the average image and performing an additional CT scan with the calculated subsequent X-Ray dose to obtain a new average image; and saving the average image when receiving the end instruction, wherein the image values of the ROI comprise a parameter reflecting image noise of the ROI in the initial scan image or the average image.