The invention relates to a cutting knife. The cutting knife is characterized in that a hollow segmental ring (3) is arranged at the front end of a knife body (1); a handle (2) is arranged at the rear end of the knife body (1); a sliding groove (6) leading to the segmental ring (3) is formed in the middle of the handle (2); a connecting sheet (5) connected with a blade (4) is arranged in the sliding groove (6); a buckle shifting block (7) is arranged on the connecting sheet (5) and fixes the position of the connecting sheet (5) in the sliding groove (6); the buckle shifting block (7) can control telescopic length of the blade (4) in the segmental ring (3). The cutting knife is simple in structure, convenient to operate, and very practical, has low possibility of hand injury, and is a necessary helper for working in household kitchens and bathrooms.