The invention provides a technical scheme of an RKA (relativistic klystron amplifier) output cavity with an inner conductor arranged inside. According to the scheme, the output cavity comprises a drift tube, an output cavity body, an output cavity end cover, an output coaxial outer conductor, a collector, the inner conductor, inner support rods and outer support rods, wherein the drift tube is connected with the output cavity end cover; the output coaxial outer conductor is arranged on the outer wall of the output cavity body; the collector is fixed in the output coaxial outer conductor through the outer support rods; an outer nose cone is arranged on the collector; the inner conductor is arranged at one end, close to the outer nose cone, in the collector and connected with the outer nose cone on the collector through the inner support rods. The structure of the RKA output cavity is improved, the minimum energy required when electrons arrive at an output cavity gap can be reduced, and the output microwave power and the beam wave conversion efficiency of an RKA can be improved.