A method to reduce the loss of a holecoupled resonator for a freeele ctron laser oscillator is proposed. A grating with a reflectivity of 50%-70% can be used to cover the hole . Both the extraction and diffraction losses can be reduced effectively, which is confirmed by numericals imulations using a three dimensional code with the parameters of a compact far in frared FEL oscillator built in China Academy of Engineering Physics. Furthermore,it can be used as an electron transparen t mirror to make the FEL device more compact and less expensive．