The Ultrasonic/Sonic Driller/Corer (USDC), driven by the piezoelectric system, can take rock samples in other planets because of the light weight, low energy consumption and no rotating parts. The function of piezoelectric stacks is to convert the electric power into the dynamic power of the high-frequency mechanical vibrations, so the ultrasonic horn vibrates in ultrasonic frequency. It is complicated to design the ultrasonic horn due to electromechanical coupling. Based on the ANSYS environment, the dynamic characteristics of the ultrasonic horn under the boundary conditions of electromechanical was analyzed. The modal frequency and the excitation response of the coupling field were in good agreement with the results obtained from the single mechanical structure field. The result shows that the electromechanical coupling of USDC is very weak in the process of electrical energy converting into mechanical vibrating power and can be neglected for calculation. Therefore, the decoupling method will be adopted to design the ultrasonic horn in order to simplify the design process.