A novel resonator has been proposed for the klystron that operates at mode. The electric fields of both the middle cavity and the side cavity are effectively enhanced by discretizing the field in the side cavity. The novel resonator demonstrates a higher characteristic impedance than the traditional barbell shape cavity when both cavities operate at mode. Sensitivity analysis has been performed on these two kinds of cavities. Moreover， an RF circuit in the W-band has been designed based on the proposed novel cavity. The RF circuit can achieve a high gain of 52.4 dB and a peak output power of 7.8 kW when 45 mW input power is injected. The circuit is driven by a 25 kV and 2 A pencil beam. The electronic efficiency and 3-dB bandwidth are 15.6% and 120 MHz， respectively.