Using the HFSS and MAGIC code, influences of the washer/rod structure on the high frequency characteristics and the basic operation of the wide-gap klystron amplifier (WKA) are presented. Generally, the rod structure has a great influence on the space-charge potential depression and the eigenmode resonant frequency of the cavity. Once the rods are employed, the space charge field can be nearly eliminated with only 4 washers. Moreover, the presence of the rods raises the cavity eigenmode resonant frequency by ～ 9.0% due to their comparable lumped inductance to the cavity's inherent inductance. Nevertheless, if only the cavities are tuned to the desired operation frequency, the effect of the rod size on the basic operation of the WKA is expected to be very small.