Simulations, designs and experiments of the input cavity of an L-band intense relativistic klystron amplifier are described in this paper. Influences of large coupling holes on the microwave input cavity parameters which include resonant frequency, symmetry and uniformity of the electric field in the gap region were overcomed with axial symmetric slots as fig.1. They have gotten a 6％　fundamental harmonic modulated current after the electron beams got through the input cavity gap.