Abstract: The present invention discloses a multiband resonance frequency tracking circuit applied to ultrasonic machining, including an output current-voltage phase difference direction detection circuit and a multiband resonance frequency tracking circuit, where a detection signal output end of the output current-voltage phase difference direction detection circuit is connected to a signal input end of the multiband resonance frequency tracking circuit. Through the foregoing technical solutions, in the present invention, a system detuning state is quickly determined by using a current-voltage phase difference direction detection signal outputted by a D flip-flop in a chip SN74HC74D, and a dead-time resistor between a 5th pin and a 7th pin in a chip EG3525 can be changed in a timely manner based on a detuning degree of a piezoelectric transducer, so as to change a system driving frequency to track a resonance frequency of the piezoelectric transducer and implement fast and accurate tracking.