Abstract: The low-voltage frequency-tunable terahertz gyrotron is widely used in the biomedical and spectroscopy fields. In this paper, the output power and frequency tunability of a 330 GHz ultra-low-voltage gyrotron with the traditional open resonance cavity are analyzed, meanwhile the weak beam-wave coupling problem due to the mildly relativistic electron beams in the low-voltage gyrotron is discussed. Based on this, an improved interaction circuit for a 330 GHz ultra-low-voltage gyrotron is proposed. The down-taper section on the downstream of cavity can help strengthen the amplitude of the backward wave in the cavity, and enhance the coupling between the mildly relativistic electron beam and the electron magnetic wave. Thus, the output power and frequency tuning range of the low-voltage gyrotron can be obviously increased. The simulation results indicate that even with 0.3 kV beam voltage, the output power of the gyrotron with the optimal circuit still greater than 1 W in the continuous frequency tuning range of 22 GHz, and the peak efficiency is greater than 7%.