Abstract: In the high power ultrasonic application, it is difficult to search and track frequency of ultrasonic vibration system automatically as a result of the multimodal characteristic of ultrasonic vibration system.In this paper the relations between the multimodal vibration system and the phase of voltage and current of vibration system and the value of current, and the relation between transverse size of ultrasonic probe and bandwidth of the highest peak of vibration system,are studied with experimentation. On the experimental rule, the state of resonance of vibration system is identified by phase and current.The experiment of frequency searching and tracking has been done on the ultrasonic plastic welding machine operating at 20kHz, and the result shows that the method of frequency searching and tracldng with double parameters is valid.