The invention provides a circular knife assembly and a cigarette paper cutting mechanism. The circular knife assembly comprises an inner support structure, an outer support structure, a circular knifeblade and a bearing, wherein the outer support structure is located on the radial outside of the inner support structure; the circular knife blade is fixedly installed on the outer support structure;the bearing comprises a bearing outer ring and a bearing inner ring oppositely and rotaionally arranged on the radial inner side of the bearing outer ring, wherein the bearing inner ring is installedon the inner support structure, and the bearing outer ring is installed on the outer support structure, so that the outer support structure is rotationally connected to the inner support structure through the bearing; and the cigarette paper cutting mechanism includes a cutting roller and the circular knife assembly matched with the cutting roller. After a period of operation of the circular knife assembly, if the bearing is worn or damaged, the production can be resumed only by replacing the bearing, so that the replacement of the whole circular knife assembly is avoided, and the cost of spare parts is reduced while the quality of cigarettes is ensured.