The invention provides a device used for generating high efficiency narrow linewidth intermediate infrared laser and a method. The device comprises a pumping source, an OPO and two stages of OPAs. The device also comprises a coupling lens used for making idler frequency light and pumping light coincide and converge in a nonlinear crystal, a dichroic mirror used for separating signal light and the idler frequency light, and a dichroic mirror used for separating the idler frequency light and the pumping light. The invention also provides the method of obtaining narrow linewidth intermediate infrared laser output without inserting a linewidth controller in the cavity of the OPO. The method is characterized by adjusting the pumping spot size of crystals to control the divergence angle thereof, applying proper pumping power to obtain narrow linewidth seed light output, selecting the proper pumping power and pumping spot size as well in an amplifying process, and reasonably distributing the pumping power between two stages to obtain high power narrow linewidth intermediate infrared output.