A cutting device with an auto-retractable blade, having a housing containing an active blade extendable from a front aperture thereof and a spare blade box therein. An actuator is provided being fixed to an actuator seat contained in and slidable with respect to the housing. The actuator seat has a side protrusion having a front dent defined therein. A blade seat is slidably contained in the housing. The blade seat has an upper fulcrum, a side protrusion, a side blocker, a blade installation plate, a lower fulcrum, and a blade seat return spring. The side protrusion has a front slope and a back tip which is receivable in the front dent of the actuator seat.