This paper discusses the space-charge-limited current and its energy distribution in the coaxial drifting pipe. In order to bypass the potential depression problem, the triaxial output cavity of relativistic keystron amplifier was implemented. The triaxial output cavity was designed and simulated. A hollow electron beam of 590kV, 5kA generates 1GW rf power in S band klystron amplifier using the triaxial output cavity. The efficiency is about 35%, which is 9% bigger than that of the coaxial output cavity. The results agree well with the experiment.