Abstract: The output power of broadband helix TWT often form so-called power hole around a frequency. The existing power hole can affect the performance of the traveling wave tube seriously. Theoretical analysis shows that the power hole can be improved by changing the position of the attenuator of the TWT. The relationship between the attenuator’s position and the output power was obtained by the large signal calculation to increasing the power of power hole and improving the output performance of the TWT.