A utility knife with a locknut includes a case, a blade holder, a blade, a movable pushbutton and a locking device. A blade extension/retraction opening is formed at a front end of the case and provided for the blade to extend or retract, and a movable chute is formed on a side of the case and provided for the movable pushbutton to move. The blade is correspondingly installed in the case by the blade holder, the blade controls its extension and retraction in the blade extension/retraction opening by the movable pushbutton. The locking device includes a locking rod and an adjusting device and uses the adjusting device to adjust the locking rod and lock the blade.