Abstract: A small signal theory——space charge wave physics relevant to modulation properties of intense relativistic electron beams in klystron amplifiers is described in this paper. The properties are the dispersion equation of IREBs drifting in the tube, and the transmission line model of IREBs interacting with the gap electrical field of input cavity of RKA and bunching in the drift tube. The expression of modulated current after the gap of input cavity is got, which is content with the experimental results. In experiment,a voltage 500 kV,current 3 kA, pulse width 1.4 μs hollow electron beam modulated by 70 kW input microwave power generates 9.8% modulated current, and it is content with the results of theory.