Since longitudinal vibration is coupled with the middle disk in a folded acoustic horn,the conventional design method of longitudinal acoustic horn is no longer applicable to it.According to the structure characteristics and the vibration propagating feature of the folded acoustic horn,the change of modal forms of longitudinal wave in the folded acoustic horn was analyzed.The vibration model of the disk is mainly the tangential vibration which is similar to the cylindrical radial wave propagation.It converts to longitudinal wave after coupling.Based on the wave equation,the frequency equation of the folded acoustic horn was built and a method for designing the horn dimension was presented using the frequency equation and finite element analysis.The experimental result shows that the horn frequency and amplitude ratio are basically consistent with the theoretical calculation result,and the mechanical impedance is small.