The influence factor and the radiation field of the resonant frequency of the longitudinal bending composite transducer composed of a sandwich transducer and a rectangular plate are analyzed by using the finite element method. The results show that the geometrical size of rectangular plate increases with the increase of the contact area, and the resonant frequency increases with the increase of the thickness, and the frequency changes with the increase of the thickness and the frequency of the rectangular plate increases with the increase of the thickness. Study on radiated sound field results show that with increasing frequency, the modal frequency directivity peak becomes large and increasingly sharp and outside the main peak appeared more peak. The conclusion can provide theoretical reference for the practical engineering application of the longitudinal bending vibration transducer and frequency adjustment basis.