Aiming at the problem of the natural frequency of the piezoelectric ultrasonic transducer in the ultrasonic therapy apparatus that causes the factory error of the natural frequency due to the manufacturing process,and the frequency drift caused by temperature changes and other factors, it will work in a non-resonant state, which will greatly reduce the electro-acoustic conversion efficiency of the piezoelectric ultrasonic transducer, and the treatment waveform will change and the output power will be unstable, which will affect treatment effect of the ultrasonic therapeutic apparatus. An automatic tracking frequency locking circuit of ultrasonic therapeutic apparatus is designed.The system uses STM32F103ZET6 as a microcontroller. Using the principle of inverse piezoelectric effect of piezoelectric ultrasonic transducer and the maximum voltage feedback method, the maximum voltage value and the resonant frequency are fed back in real time through frequency sweep sampling, so that the piezoelectric ultrasonic transducer always works at the resonance point, so that the To achieve automatic tracking frequency locking. The experimental test proves that the ultrasonic therapeutic apparatus based on the automatic tracking frequency locking circuit can maintain a relatively stable output power, a good treatment waveform, accurate frequency tracking and low cost, and can achieve a good treatment effect. Therefore, this subject has better innovation, application prospect and broad applicable value.