A tunable acousto-optic Q-switched CO2 laser was presented using a blazed grating. The tuning principle of CO2 laser was analyzed, and it was found that changing the intra-cavity loss of lines is an effective way to realize the laser wavelength tuning. Then the relationship between diffraction efficiency of blazed grating and the laser wavelength were researched. The theoretical calculations show that the grating has highest diffraction efficiency when the auto-collimating angle of laser line equals to blaze angle of grating. Two blazed gratings with blaze angle of 31.97° (blaze wavelength 10.59 μm) and 28.71°(blaze wavelength 9.60 μm) were employed and the tuning performances of acousto-optic Q-switched CO2 laser were researched on experiments. Laser emission lines of 65 and 75 were obtained separately. The emission lines were richer when blaze wavelength of grating equals to the laser wavelength with lower gain, which accorded with the theoretical calculations. When the repetition frequency is 1 kHz, the maximum average power of 4.2 W, pulse width of 160 ns, peak power of 26.25 kW are achieved at the wavelength of 10.59 μm, showing good stability.