A new cavity construction with coaxial-off grating has been proposed on the basis of waveguide cavity theory. An extra-cavity waveguide laser has successfully developed with plane grating of selecting spectral branch. Its behaviour is equivalent to a close cavity with the concave grating. Tuning range of the waveguide laser is from 9.2μ to 10.8μ, and the oscillation at four bands of CO2 has been obtained. More than 57 spectral lines have a power above 0.5 W at a single line. The maximum output power at a single line. The maximum output power at a single line is 2.32 W.