Abstract: The relevant facts to bandwidth of relativistic klystron amplifier (RKA) are analyzed in this paper. The RKA is simulated by PIC to analysis the bandwidth relevant to perveance, the size of electron beam, the parameters of the cavity. It found that bandwidth is mainly relativity to the modulation current and the Q of the output cavity. The result of the simulation shows that the bandwidth is change with the voltage, current, the size of output cavity. And it has the maximum bandwidth as frequency of the middle cavity is the same with the best input frequency. The size of the electron beam has no effect on bandwidth.