With numerical calculation and particle simulation program, the influences of the intense electron beam impedance, voltage and current characteristics on the beam modulation and bunching characteristics in relativistic klystron amplifier (RKA) are analyzed. Within the particle-in-cell simulation program, the beam emission method is used to accurately control the impedance of the electron beam. The results show that the electron beam of low-impedance can reduce the bunching distance and shorten the overall length of the RKA devices but degrade the injected modulation of the electron beam. Electron beam of high impedance is just opposite. When the electron beam impedance is constant, the increase of the electron beam accelerating voltage is similar to the increase of the impedance of the electron beam. In addition, with the particle simulation method, the beam loaded conductance of a specific input cavity loaded by a different impedance of the electron beam is determined, thereby meeting the demand of the power level of the seed, and the requirements for the externally loaded quality factor of the input cavity.