Abstract: The Recirculating Coaxial Resonator Cavity Electron Accelerator is a reentrant electron accelerator mainly used for industrial irradiation processing. In its radio frequency (RF) power transmission system, the final-stage power amplifier utilizes a TH781 tetrode, and the amplifier’s anode output cavity is directly connected to the acceleration cavity via a coaxial coupler. Based on CST Microwave Studio, this paper systematically conducts simulation optimization on the final-stage power amplifier of the accelerator’s power source system, determining the optimal height of the adjustable short-circuit surface of the output cavity. For two accelerators with different energy levels (10 MeV with a single-coupler layout and 40 MeV with a three-coupler layout), the optimal coupler parameters enabling efficient RF power feeding into the cavity are obtained, including the optimal coupler position and the optimal rotation angle of the coupling loop under critical and over-coupling conditions. Additionally, simulations of the secondary electron effect in the coaxial coupler are carried out, identifying the locations prone to secondary electron multiplication and the corresponding RF power range. The results show that the secondary electron yield (SEY) of the ceramic window is less than 1 within the power range of 0 ~ 9.2 kW, and only 0.997 under the full-load condition of 200 kW, presenting no risk of damage. This research provides key technical references for the design, installation, commissioning, and practical operation of this type of accelerator.