Aimed at the terahertz G-band, a high-power dual-voltage-mode Traveling-Wave Tube (TWT) high-frequency circuit is designed. By employing two selectable beam voltages, the structure can be driven either in a high-power pulsed mode or in a low-power continuous-wave mode, endowing the TWT with true dual-mode operation. When combined with arc-curved-boundary folded-waveguide sections and a phase-velocity tapering technique, this design markedly boosts the output power up to 130 W in the high-power mode and 52 W in the low-power mode, while both modes achieve a 3-dB bandwidth of 11 GHz. The developed dual-voltage-mode G-band TWT meets the practical requirements for dual-mode sources in electronic-warfare and communication systems, and its high power and broad bandwidth make it a promising radiation source for terahertz applications.