A knife allowing rapid and safe replacement and securement of a blade which can be fixed to a front end of a connecting base of the knife. The connecting base has a rear end that can be connected to a handle, and includes an upper base body, a lower base body, a lock-release member, and a spacer plate that can be located between the upper and lower base bodies and form an engaged-connection groove along with the base bodies therebetween. The engaged-connection groove matches, and enables the insertion therein and engagement therewith, a portion of the blade at or adjacent to the rear end thereof. The lock-release member can be arranged on the connecting base for a user to set the blade into a locked state, in which the blade is securely positioned and cannot be detached, or an unlocked state, in which the blade can be detached.