Abstract: Ultrasonic horn is an important part in vibration system of ultrasonic plastic welding equipment,and its performances have directly effect on welding quality.In this paper,the strip-like horn is separated into several equal portions,and each portion can be viewed as a compound stepped type horn,and its frequency equation is derived by the transmission matrix method.Experiments show that the measured resonant frequencies of strip-like horns consist very well with the calculated results based on the frequency equation.The physical meaning of this design theory is clear and its calculation is simple,so it is fit for engineering design.Moreover,the relations between the slot number,the slot wide or length and the dimensions of horn can be easily derived from this design method,hence,a theoretical basis for the opti-mization design of strip-like horn is provided to a certain extent.