A protection circuit is coupled to a switch circuit of a machine tool for protection. The protection circuit comprises a fatigue protection unit and a misoperation protection unit. The fatigue protection unit comprises a first sensor, a first processor, a first controller, and an alarm module. The misoperation protection unit comprises a second sensor, a second processor, and a second controller. The protection circuit senses the degree of fatigue and the misoperation of people through the fatigue protection unit and the misoperation protection unit, then controls the machine tool to work or not to work to avoid accidents.