Abstract: A FIR –FEL resonator with wavelength 500m m is studied, which has a semi –closed optical cavity composed of a plane waveguide and two symmetric cylindrical metal mirrors. There is a hole of radius Rh on axis in the down –stream mirror. Some three-dimensional numerical results are given, which provide the interactive power, the output power, the extraction ratios and the diffraction loses on the mirror edge for various hole radius. If chewing an appropriate hole, so interactive power and good cavity modes can be obtained, and outputting the power very effectively.