The invention provides a protecting device of an ultrasonic cutting tool and a protecting method. A torsion protecting device is added between a belt wheel for connecting the ultrasonic cutting tool and an output shaft of a motor. When an abnormal condition occurs to a stacking, an inner sleeve and an outer sleeve of the protecting device are disengaged from each other so that the motor is in idling running, furthermore, the ultrasonic cutting tool is stopped moving forward and vibrating through the belt wheel. Therefore, a device and method for detecting and controlling the working state of the ultrasonic cutting tool immediately is realized. The asynchrony of the motor and the running of the belt wheel is realized through the resistance of the stacking to the ultrasonic cutting tool, so as to trigger a detecting switch and control the ultrasonic cutting tool directly and drive the motor to stop working.