Abstract: A simple and effective method based on boundary element method, is proposed for computer determination of the resonant frequency and field distribution of the waveguide-loaded cavity in klystron output system. This paper calculates a common rectangular-waveguide-loaded rectangular cavity. The TE10 mode wave is used as the input wave, and the Robin-type boundary condition is used in the output waveport. The maximum of the electric field is searched, which corresponds to the eigenfrequency. The obtained eigenfrequency is lower than that of the cavity of the same size and loaded with no waveguide, and the field distribution is under the influence of coupling hole. It is shown that this method needs less computing time and resources in solving the half-open system like a waveguide-loaded cavity with high precision.