Abstract: This paper focuses on the research of a mode converter for power transmission in the S-band 200 MW peak power klystron. Considering the power capacity issue, the high peak power klystron adopts two-way output structure and achieves the mode conversion from rectangular waveguide TE₁₀ to circular wave guide TE₀₁ by using a side coupled structure mode converter, when the standing wave ratio is below 1.05, the field strength at the output window is reduced from 3.46×10⁶ V/m to 1.40×10⁶ V/m, and the mode conversion rate at the center frequency reaches 99.94%. Compared with traditional Y-branch power dividers, abandoning the Y-branch at the power divider and sacrificing some bandwidth to achieve the effect of reducing field strength and increasing power capacity. And a petal mode converter is designed. By comparison, while considering both standing wave and conversion efficiency, the side coupled mode converter has a lower field strength and a stronger suppression effect on stray modes. This design can be extended and applied to other frequency bands of peak power klystrons, which is of great significance for the design of future peak power klystron products.