Abstract: Using three-dimensional PIC software, the first time establish a klystron output cavity external rectangular waveguide coupling device geometry is fully consistent calculation model, the output of the S-band rectangular waveguide cavity loaded to do the design and optimization, in the working voltage 80kV, operating current 45A, the work under the conditions of the center frequency of 2.85GHz, has been 1.43MW output power, 3dB bandwidth of 6% In the experiment, theoretical simulation and experimental results are completely consistent.