Abstract: The optimum design method for the ultrasonic transducer in the ultrasonic-arc hybrid welding system is presented in this paper.The whole structure of the transducer and the material are chosen by the work conditions.On the basis of the traditional analytical method,the resonant frequency of the whole ultrasonic transducer is deduced in order to get the basic dimensions.The modal of the ultrasonic transducer is analyzed by the finite element analysis with ANSYS.The effects of the front block size and the back block size to the ultrasonic transducer resonant frequency are researched and further optimize dimensions of the ultrasonic transducer is approached.With an impedance analyzer,the impedance characteristics of the ultrasonic transducer is tested and results show that the resonant frequency of the ultrasonic transducer is basically consistent with the modal analysis of the ANSYS and the main performance parameters are good.