In this paper the question concerning matching of CO2 laser beam with open resonator of a far infrared laser was analysed, and the quantitive relation between the absorptivity of working material of the far infrared laser and the outgoing loss of coupling hole was given. It was shown that there is an optimal divergent angle for a given resonant cavity of a far infrared laser. At this angle active volume of the working material was maximized, and higher output of the far infrared laser was obtained.