According to the research needs of ultrasonic coupled contactless energy transfer (UCCET) system, the transmitting transducer of underwater UCCET system is selected, and the type and operating frequency of the transducer are determined. Based on the one-dimensional design theory of the sandwiched longitudinal vibration piezoelectric transducer, the design of transmitting transducer is optimized. On the basis of the in-depth analysis of the influence of the extension coefficient of the conical front cover plate on the vibration velocity ratio and the equivalent electromechanical coupling coefficient of the transducer, the selected extension coefficient can simultaneously take into account the vibration velocity ratio and the equivalent electromechanical coupling coefficient of the transducer as the optimal value, then the overall structure size of the transducer is further determined. The actual test results show that the actual resonant frequency of the trial-produced transmitting transducer is 38.552 kHz, and the equivalent electromechanical coefficient is 0.149, which can meet the practical needs.