In this paper, the two-layer 1-3 composite transducer using an unconventional part-excitation method is presented to achieve the ultra-wide operation band for the high-frequency underwater devices, in which the part-excitation refers to the method that only the lower-layer is excited, while the upper-layer in contact with the propagation medium is not excited. This method is first proposed by analyzing the longitudinal vibration modes of one piezoelectric rod in the 1-3 composite and finally, we conclude that if the excited lower-layer can be controlled to have thickness given by 3l/2n, where l is the whole thickness of the two-layer rod and n (n > 1) denotes the number of the coupled modes, then first n order modes can be activated simultaneously to form an ultra-wide operation band containing n response peaks. Finite element method (FEM) is employed to investigate the two-layer 1-3 composite with a thickness of 10 mm. The results show that when the thickness of the excited lower-layer is 3 mm (n = 5), the transducer attains the ultra-wideband by coupling five resonances, nearly covering the frequency range from 100 kHz to 1 MHz. According to the difference of response level, the whole band can be divided into multiple local wide bands. A prototype transducer is fabricated based on the FEM results, and the measured results show that the whole band contains three wide bands, covering the frequency ranges 125-295 kHz, 300-480 kHz, and 500-915 kHz, respectively.