Abstract: A retractable knife, having a handle formed with an accommodating space which has an opening at one end of the handle; a side surface of the handle has a first sliding groove communicated with the accommodating space and a first rack corresponding to the first sliding groove; a blade is movably mounted in the accommodating space through the opening of the accommodating space; a second rack is arranged on the blade; a push button assembly is movably mounted on the first sliding groove and connected to the blade; the push button assembly can push the blade to move; the push button assembly has a gear engaged with the first rack and the second rack to form a speed enhancement transmission mechanism. A longer stroke of the blade during discharge and withdrawal can be achieved by a shorter stroke of the push button, thereby facilitating one-hand operation for convenient use.