Abstract: In order to meet the power source requirements of C-band linear accelerators for industrial applications, a C-band high power klystron operating at a frequency of 5 712 MHz was developed. For this klystron, the development objective of the output power is no less than 3.5 MW and the efficiency is no less than 35%. During the entire design process, the output power was optimized to over 4 MW and the power efficiency over 40% to compensate for the disadvantage effects caused by manufacturing and testing errors in the development processes. Firstly, the electron optics system with a Pierce gun and a combination of five forward and one reverse focusing coils was designed. The Pierce gun with a perveance of 2 μP was adopted and the beam voltage and current were chosen as 120 kV and 83.3 A. By iteratively calculating the structure of the electron gun with EGUN code and optimizing the magnetic field between the electron gun and the beam-wave interaction section, the final electron beam pass rate reaches 100% and the fluctuation of the beam envelop is only 4.8%. Secondly, the efficiency optimization and PIC simulation of the beam-wave interaction section were carried out using the one dimensional AJDISK code, 1.5 dimensional KlyC code and three dimensional CST code. The scheme of six resonant cavities was ultimately determined and the basic cavity parameters were optimized based on the principles of stagger tuning. The simulation results of different codes all indicate that the efficiency can reach above 46.5%, and the output power can all reach above 4.650 MW. And the influence of frequency errors and output external quality factor on efficiency was also analyzed to guide the manufacturing process. After that, the mechanical structure of cavities was designed and all cavities were manufactured with oxygen free copper. The cold test and tuning of all cavities were carried out and the frequency error is mostly within the range of ±5 MHz, which basically does not affect the output power. Finally, the electron gun, the beam-wave interaction section and the collector were welded together. And the high power test was conducted immediately after the first high power C-band klystron protype was completed. When the electron gun operates at the voltage of 115 kV and current of 79.6 A, the klystron peak output power reaches 3.52 MW, the gain reaches 45.5 dB and the efficiency reaches about 38.5%. The high power test results show that this C-band high power klystron achieves the expected development goals.