1. A rotating-grating cone beam CT imaging apparatus, characterized in that said apparatus is composed of a frame and a drive device thereof, a frame angle sensor, an X-ray source equipped with a rotating grating, a flat panel detector, a main controller and an image reconstruction workstation, wherein said X-ray source is mounted on the frame, and rotates together with the frame in the process of collecting projection images, and while the X-ray source rotates together with the frame, said rotating grating also rotates around the X-ray source, so that projection images of narrow-angle cone beams or sector beams passing through the gratings scan said entire flat panel detector, and this scanning process is repeated continually; and said rotating grating is composed of several groups of gratings located at different rotation angles, said several groups of gratings extend and are distributed in length in the circumferential direction of said rotating grating, and as compared with the slits of the previous group of gratings in said several groups of gratings, the slits of the next group of gratings axially offset along said rotating grating entirely, so that when said rotating grating rotates, the positions of the narrow-angle cone beams or sector beams passing through the gratings can be changed.