Abstract: This paper presents high frequency characteristics analysis on an open input cavity of an S-band relativistic klystron amplifier and the optimization of the cavity geometry. Cold test on the high frequency parameters of the fabricated cavity is performed, and experimental results agree well with that of high frequency analysis. A three-dimension fully electromagnetic particle-in-cell(PIC) code is used to simulate the absorption of the injected microwave and the fundamental harmonic modulated current while the electron beams propagating the input cavity gap. The simulated results show that the cavity matches with the input waveguide and the whole energy of the driven microwave is absorbed by electron beams. The depth of the fundamental harmonic modulated current is about 13% at 37 cm downst