The invention provides a rotary ultrasonic machining driver based on a BUCK circuit. The driver comprises a single-chip microcomputer, a power supply circuit, an ultrasonic transducer, a voltage and current sampling circuit, a phase lock circuit, a high-frequency transformer, a non-contact power transmission device, a full-bridge inversion circuit, a BUCK direct current chopper circuit and a driving circuit, wherein the single-chip microcomputer outputs a BUCK driving signal and a driving signal of the full-bridge inversion circuit by an IO (Input/Output) port to respectively control the BUCK direct current chopper circuit and the full-bridge inversion circuit; the BUCK direct current chopper circuit is connected with the full-bridge inversion circuit; the full-bridge inversion circuit outputs a signal to the high-frequency transformer; the high-frequency transformer is connected with the ultrasonic transducer by the non-contact power transmission device; and the voltage and current sampling circuit acquires voltage and current of an output signal of the high-frequency transformer, and then feeds back to the single-chip microcomputer by the phase lock circuit. The single-chip microcomputer achieves small-range adjustment of a duty ratio of a driving signal of the BUCK direct current chopper circuit by feedback signal processing, and power adjustment of the ultrasonic machining driver during a machining course is achieved by changing the duty ratio of the driving signal of the BUCK direct current chopper circuit and adjusting output voltage of the BUCK direct current chopper circuit.