Abstract: With the 2.5D full electromagnetic particle-in-cell code, an S-band inductively-loaded wide-gap cavity relativistic klystron amplifier is proposed and investigated. The effects of the washers/rods parameters on intense relativistic electron beam modulation are given. Simulation results show that wide-gap cavity can extend beam/cavity interacting duration and increase the efficiency. The increase in potential energy of wide gap can be prevented by the washers/rods return current structure. Making use of a 500 keV, 6 kA electron beam, about 4.5 kA modulation current and a microwave with power of 1.2 GW and efficiency of 40% are obtained.