Abstract: In the basis of theoretical analysis,some fundamental four-terminal network matrices and frequency equations of variable cross-sectional bar on various motion state are given in order to investigate convenient and precise designing method,and a new design method of piezoelectric transducer used in ultrasonic welding is studied.The piezoelectric transducer and the variable amplitude bar are designed by four-terminal network method.The resonant characteristics are analyzed and the vibration performance is measured for the vibration apparatus with an impedance analyzer.The results show that the analysis and the design are correct.