The resonance characteristics of ultrasonic transducer are analyzed based on the equivalent circuit of transducer and the quantitative calculation of admittance circle, and a control method of frequency tracking based on variable step length is proposed, and the limits of step switching are deduced, and the control parameters of the control method based on variable step length is designed in this paper. The results of simulation and experiment show that the system based on the control method can control output frequency in high precision. And while ultrasonic power supply ensure steady-state error minimum, holding a good dynamic performance, which meets the requirements of the technology of ultrasonic power supply.