Abstract: In the resonant system of ultrasonic transducer, the traditional frequency tracking method is unsatisfactory in real time due to the long startup time and the narrow operating frequency bandwidth. Therefore, a frequency tracking method for the transducer is proposed by real-time monitoring the phase angle changes of the current and voltage in the LC matching network to reflect the resonance frequency misalignment of the system and then by changing the phase angle to match the resonance frequency of the system and realize frequency tracking of the transducer. The fuzzy proportional integral(PI ) control idea is used to improve the fixed gain PI control strategy, which reduces the amount of overshoot and the system response time. Online simulation of fuzzy PI is carried out by using Matlab, the phase angle and its change rate over time are taken as the fuzzy PI input and the resonance frequency as the output. The fuzzy PI outputs the K_p and K_i changes required by the PI controller to realize adaptive changes of the PI controller output and the resonant frequency. Simulation and experimental results show that the current and voltage phase angles can be traced to 0° in effective time, the overshoot of PI control can be effectively reduced, and this method enables the whole system to achieve better frequency tracking effect.