The present invention provides an unstable resonator selected output method of CO2 laser and belongs to the technical field of high-energy CO2. The method uses a plating technique for respectively executing the plating of different characteristic curves to two spherical mirrors. When laser oscillates and amplifies for competing output in virtual confocal positive-branch unstable resonator, one spherical mirror restrains the oscillator amplification of laser with central wavelength of 9.6 mu m. The other spherical mirror restrains the oscillator amplification of laser with central wavelength of10.6 mu m. Thereby the output of laser with central wavelength of 9.3 mu m is realized. The unstable resonator selected output method of CO* laser according to the invention has the outstanding characteristic of excellent practicality and good effect. The high-quality quasi-fundamental mode output light beam which is close to the diffraction limit can be obtained. The laser beam with higher energy can be output. The light beam with more homogeneous energy distribution can be provided.