Abstract: As a high-frequency interaction circuit of the klystron, the characteristics of the resonator have a decisive influence on the power, efficiency, gain and bandwidth of the klystron. This paper mainly introduces the design process and analysis of a Ka-band extended interaction klystron resonant cavity. Based on the multi-gap resonant cavity theory, the electromagnetic simulation software CST is used to analyze the influence of different structural dimensions of the resonant cavity on its characteristic parameters, such as quality factor, characteristic impedance, coupling coefficient and effective characteristic impedance. The physical structure model of the five-gap resonant cavity with a resonant frequency of 35 GHz is optimized, and the interaction simulation results are given, which provides an important reference and basis for the design of Ka-band distributed-action klystron and the calculation of high-frequency beam-wave interaction.