Abstract: This paper presents a 3-D model for double-gap open cavity of S-band relativistic klystron amplifier. By observing the response of the current driver, the high frequency characteristics including resonant frequency, loaded quality factor (Q), field distribution and character impedance of the open cavity are calculated. The effect of the open cavity size on the resonant frequency, loaded Q value and the character impedance are discussed. The results show that the frequency mainly depends on the radius of the cavity and the loaded Q value diminishs owing to the increase of the angle of the coupling hole, and the character impedance declines due to the increase of the radius and the decrease of the gap length of the cavity. The analysis provides a theoretical basis for the design of the doubl