Abstract: A C-band relativistic traveling-wave-structure amplifier is designed from dispersion relation of disk-loaded slow-wave structure. In order to avoid self-excitation of the amplifier, some methods are employed in the tube design as follows: coaxial extraction to suppress reflection, sever and absorber to reduce substantially positive feedback, tapering output to reduce retrograde electrons. On the basis of cold cavity characteristic research, quantitative analysis of beam-wave interaction is performed using 2D PIC simulation code. The highest output power, the gain and efficiency is about 890 MW, 35.6 dB and 32% respectively for a 5.64 GHz, 250 kW input.