Abstract: In order to extend the working band of the underwater transducer and to solve the problem of poor coupling of some modes in the multimode coupling transducers, the effect of the phase in the vibration modes of the transducer on the modal coupling capability is studied by using the equivalent circuit method. The methods for obtaining and controlling the phases in vibration modes and improving the coupling capability are presented. The phase characteristics of the first and second order vibration modes of the class-IV flextensional transducer are mainly analyzed. A cavity structure can be added to the ellipsoidal shell of class-IV flextensional transducer to adjust the phase of the second order vibration mode in the flextensional transducer, thus constructing a wideband class-IV flextensional transducer by using the coupling of the first and second order flexural tension vibration modes. A prototype of a wideband class-IV flextensional transducer is fabricated with a working band of 2–7 kHz. The effective coupling of the first order and the second order flexural tension vibration modes is realized, and the effectiveness of the proposed method is verified.