After investigating the multi mode wideband composite transducer, it has been found that the flexural and longitudinal vibration of this kind of transducer in air has significant regularity. For 1 3 type piezo composite material used as the piezoelectric stack, the effect of the volume fraction of piezoelectric ceramic on the resonant frequency has been studied by using the commercial finite element method and the relationship between the resonant frequency of the transducer and its dimension has also been studied in this paper. The results show that the resonant frequency of transducer will become higher with increase of volume fraction; and will become lower with increase of the dimension of front layer of transducer, and the resonant frequency is lower, the interval of longitudinal and flexural fundamental frequencies will be smaller.