Considering the effect of the position of the laser axial-modes on the gain shape and the pull-in frequency, we have calculated the wavelength-tuned characteristics of the off-axial grating cavity for vibration-rotation transition in CO laser when rotating the grating. It has been found that more transition lines can be selected by the off-axial grating cavity, and the optimum output can be obtained with the off-axial cavity. The effect has been demonstrated in the experiment.