Abstract: Klystron is a core vacuum electronic device for high-power microwave systems and is widely applied in fields such as particle accelerators, controlled nuclear fusion, and radar communication. Its key performance indicators including output power, gain and efficiency are mainly determined by the interaction efficiency between the electron beam and the high-frequency field. To meet the application demand of high-performance klystrons for high-quality electron beams with low fluctuation rate, a study on the design and optimization of the magnetic field and electron optical structure in the transition region of an applied C-band high-power klystron was carried out. The structural design of the electron gun was completed by the synthesis method, a magnetic field calculation model for the transition region was established, and the requirements for the electromagnetic field distribution in the transition region were proposed. Combined with the electron beam envelope of the electron gun, the fluctuation rate of the whole klystron was calculated. A multi-physics field simulation model is constructed using software such as POISSON and CST, and a reverse coil structure is adopted to achieve precise magnetic field matching. The reliability of the calculation results is verified through cross-validation of 2D and 3D simulation results.The final calculated beam ripple is 4.32%. This design method provides important theoretical support and practical reference for the high-performance design of the magnetic field in the transition region of high-power klystrons.