Abstract: In order to meet the requirements of the power source in C-band linear accelerator for industrial applications, a C-band high-power klystron was designed in this paper. The klystron mainly comprises an electron optics system, a beam-wave interaction section, and a collector. The required operating frequency is 5 712 MHz, and the output power is no less than 4 MW. Each part of the klystron has been optimized separately followed by an overall simulation of the entire tube. Firstly, the electron optics system was designed and the Pierce electron gun with a perveance of 2 μP was adopted. After analyzing the basic parameters, the beam voltage and current were chosen as 120 kV and 83.3 A. The Pierce electron gun was initially obtained by using the comprehensive method and the structure size parameters were optimized and determined using the EGUN program. The focusing system was composed of five forward coils and one reverse coil, and it’s convenient to adjust the magnetic field distribution in the electron optics system and optimize the beam transition. By iteratively adjusting the magnetic field distribution in the transition region between the electron gun and the beam-wave interaction section, the final electron beam pass rate reaches 100%, and the fluctuation of the beam envelope is only 4.8%. Secondly, the beam-wave interaction section composed of six resonant cavities has been optimized using the one-dimensional program AJDISK and the twodimensional program KlyC. The basic parameters of the resonant cavities were determined based on the principles of stagger tuning, and the input coaxial coupling ring structure and the output waveguide structure were designed to match the external quality factor requirements. The results show that the output power could reach 4.86 MW with an input power of 100 W, and the efficiency was 48.6%. After that, the collector was designed and a groove-type double-layer water jacket structure was adopted. It includes cylindrical, and conical parts containing 27 and 10 transverse grooves for water cooling and heat dissipation. The collector will absorb all the beam power when the klystron has no output. The average power consumption density on the inner surface of the collector is about 31 W/cm² at a duty ratio of 2‰. The mass flow rate is 1.66 kg/s at a flow rate of 4 m/s, and the maximum temperature on the wall of the collector is about 88 ℃. Finally, the entire tube PIC simulation was carried out after all parts were designed. The simulation results show that this C-band high-power klystron can stably output 4.65 MW power at 5 712 MHz with an efficiency of 46.5% when inputting 100 W power.